The brake pedal on my 914-6 is very spongy, even after repeated bleeding with a pressure bleeder. I isolated the problem to the brake-proportioning valve or pressure regulator. I cannot seem to get the brakes properly bled with this valve in the system.

I've read other posts by some here and it seems this valve is a source of some debate . Is there some trick to bleeding the brakes with it? Do I need to bleed the brakes the old-fashioned way, have someone push on the brake pedal?

No trick concerning the proportioning valve. . . all that is inside of it is a sliding piston and a big spring. . . the piston moves and cuts off brake fluid to the rear calipers when a preset pressure is exerted. .
When I redid my entire brake system, I couldn't successfully bleed it with a pressure or vacuum single person system even after a dozen tries. . . . finally had a friend come over and and we did it the old way still took going all the way around 4 times to get a rock solid pedal. .
What I finally figured out was when cracking the bleeders loose to bleed, air was seeping back into the calipers from the bleeder threads. .

Thanks for your input toolguy. I guess I will try to find someone to push on the pedal. I'm not sure I understand your comment about air seeping past the bleeder threads though. If I take the valve out of the system I can bleed the brakes fine, which would seem to exclude air seeping past the bleeders.

Everybody needs these: http://speedbleeder.com/

Air seeping past the bleeder threads on the calipers. . . this was when i tried using a Mity-Vac. . no matter how many times I bled them, I keep getting air bubbles. . because the Vac was sucking air past the thread as well a fluid from the caliper. .

Next thing is to make sure your partner understands what "Pump it up" and "Hold the pedal" means until you get the bleeder tightened. . that's been the cause of many arguments when you get the wife to help .

Removing the compensating valve isn't a good idea. . It's purpose is to keep the back brakes from locking up under very hard braking . . As you apply hard brakes, the cars weight is transferred to the front and the rear end get's light. . without the valve, the rears tend to lock up and the resulting fishtailing is unpredictable especially at high speed. .

The amount of rear braking can be adjusted by the external allen bolt, which controls inside spring preset pressure. . It's preset at the factory and not a normal adjustment without measuring inside hydraulic pressure. The compensator factory preset is different from Six's and Four's.

+1

If you don't mind the super-fund clean-up mess, crack every connection like it was a bleeder valve. Crack a connector, pump and hold, tighten connector then let off pedal...repeat.
Hell of a mess all over the floor, but it works every time.

Don't remove the valve. You will eventually come in contact with Eric Shea and he will poke your eyes out.

Speed bleeders (above). I did my '70 4 cyl last summer. Took about four trips around the car, one valve at a time. You can do it by your self.

+2 ....

After you think you are done, drive it and slam the brakes a few times...the last bubble will get through, then bleed the rear one more time....you will be done.

Thanks for all the advice guys.

Mark, besides being messy, what do I do about getting brake fluid all over the painted parts? Can't imagine that would be good.

I had no thoughts of eliminating the valve. I have been giving some thoughts to replacing it with one of the aftermarket valves. But I'll persevere.

-Damon

There is no aftermarket valve.

It is a "pressure regulator" as you've stated in your title. The aftermarket ones are bias controls, reducing fluid flow to your rear calipers. Not a good idea if you have a properly biased and designed system from the factory (1.6:1 ratio).

Here's a great read:

Click to view attachment

Readers Digest: This spring loaded valve will slide over under pressure allowing a micro switch/check valve to shut off fluid to the rear calipers in a panic stop (525psi). Once the pressure equalizes in the system this valve brings the rears back into the circuit.

One of the first anti-lock brake devices. Meant to save a 914 from spinning (like a top) with it's mid-engine and polar moment inertia.

Did you recently rebuild the calipers? If so, the new seals are causing the pistons to pull away from the rotor at a more aggressive rate. Bed the pads and the seals will get broken in by the heat. After a while, the pedal will begin to come back up.

If not, you probably have some residual air in the system. Bed the pads and then re-bleed the rears after driving the car (bumps, rail-road tracks, etc.) All of these things help dislodge air bubbles that may be trapped in the system.

Do you have the venting clearance set on the rear calipers? If not, you'll want to set that at .004"

***IMPORTANT*** We have been finding new rear pads to be thicker than original spec. If you have new rear pads, check and make sure they are 13mm thick. Some companies have begun adding a thin metal backing plate. This will cause the need for the inner piston to be hyper extended inward pulling the pushrod away from the handbrake armature. This will cause you to have a weak or non-existent handbrake no matter how much you adjust the cable.

Solution: Remove the thin metal backing plate if there is one and/or grind the pad material down to 13mm before installing them in the calipers.

I think there are other aftermarket valves available, for instance at this link here.

All these valves (proportioning, biasing, regulating, whatever they call them) effectively reduce fluid flow to the rear brakes, although the real goal is to reduce the increase in the rear brake circuit pressure with respect to the increase in the front brake circuit pressure. They don't limit the absolute pressure to the rear brakes. That's what a pressure-limiting valve does, and I've never seen one of those used in any passenger car worth driving.

This valve in the 914 was made by Bosch in response to EU requirements of the time for improved braking distribution. I think all these valves, which were developed before automotive ABS caught on, work pretty much the same way, generating a curve like the one in the page from the 914 service manual that Eric posted. For a 'cleaner' description (ie written by someone who spoke English as a first language) see this attachment lifted from "Safety, Comfort and Convenience Systems" by Robert Bosch GmbH, June 2006. This description makes it much clearer how the valve is supposed to operate.

Click to view attachment

Actually there were also load-dependent valves that connected to the suspension to sense changes in suspension deflection (see Figure 3 in the attachment) and so have two inputs (master brake pressure and vehicle attitude), I once had a FIAT 124 that had one of those. But I digress.

I've never had problems bleeding systems with these valves in them before, so I'm a little stumped about why I would be having this problem now. I don't think I'm having problems with the rear calipers, which by the way were recently rebuilt, because when I take the valve out of the system I can bleed the brakes just fine.

I really want to keep the stock valve. I'll take advantage of the advice of the group here and try to enlist the help of a friend or try the speedbleeder idea.

Thanks to all! This site is an amazing resource to have for these cars.

-Damon

There is nothing about the pressure regulator which could complicate bleeding the brake system.

If you can feel and hear the spring begin moving inside the regulator when you depress the brake pedal past a certain point then it may need adjustment to improve overall braking performance.
The adjusting screw may be more or less permanently locked in position though.
If you can get it to turn freely, I suggest removing it and replacing with a standard allen head bolt after carefully measuring its adjustment depth.
Then screw the new bolt in up to a quarter inch deeper and see if the pedal feels better.

There's 100's of bias proportioning valves available. No pressure regulators that I know of and they're quite different as the 914 is the set pressure variety discussed in the Bosh paper. That's what I meant when I said <<The aftermarket ones are bias controls, reducing fluid flow to your rear calipers.>>



So you've bled the system both ways? You put a T in and tried it without the pressure regulator?

Sounds like you still have some air in the pressure regulator.

Sorry for the late response. Work has been a bear lately.

I bled the brakes with the whole rear of the car (regulator and rear calipers) out of the brake system by replacing the line from the master cylinder to the inlet of the pressure regulator with a bleeder valve at the 90 degree connector where it comes out of the back of the firewall. When I do this I get a perfect solid pedal.

I also bled the system with only the rear brakes disconnected by replacing the hard line to the rear caliper on each suspension control arm with a bleeder valve. In this case I get a spongy pedal. On initial application of the pedal it feels soft, and if I keep applying pressure to the pedal it goes all the way to the floor, over the course of about 15 seconds.

I've tried various approaches to try and get the air out, tapping on the regulator, using speed bleeders and aggressively pushing the brake pedal, going around the car 4 or 5 times. Nothing seems to work.

So I guess I have to take out the regulator. I don't have a lift at home. It doesn't look terribly easy to get access to it. I guess I have to take out the heater controls and oil lines.

Has anyone else had this experience?

I wonder if an additional bleeder at the highest point of the system wouldn't help getting a solid pedal. I did install a T fitting many years ago (sorry, Eric), and have had a very solid pedal, but I certainly can understand the purpose of the regulator. I assume that 911s and VW Beetles didn't need these because of the higher polar moment of inertia compared to the 914 design.

I also think that flexing of the floorboars where the master cylinder is mounted also contributes to the spongy pedal feel. Eric's master cylinder brace should help address the movement quite easily (or there are posts on how to use hoseclamps and metal to achieve the same effect, but much less elegantly).

The highest point would probably be the at the top of the T-fitting coming out of the pressure regulator. There is no room above there for a bleeder, unfortunately.

Although I can see the a theoretical benefit of stiffening the master cylinder mount, in practice I don't have any problem with the stock arrangement. As I said, without the pressure regulator I get a really nice solid pedal, which means that my master cylinder is fine.

I've decided for now to replace the factory pressure regulator with one made by Tilton. Suitable adjustment will give me the same characteristic (see attachment) as the Bosch unit (which can be seen in a previous attachment). Additionally I can adjust the Tilton valve all the way open when I want to bleed the brakes (this would be possible with every adjustable prop valve out there).

I'm choosing the Tilton valve because they provide alot of technical data which lets me choose a characteristic similar to the Bosch valve, at least to start with, and it's available with metric brake line sizes so I don't have to use adapters.

Click to view attachment

Yes, old fashion way.
You need approx 750 psi to open that valve.

I do it once with a human helper, then go drive the car while going from 60-0 a few time very hard to push any remaining bubbles through the system, then when the pedal goes Dow to the floor again, I bleed it once more.

Wrong move. All the Tilton can do is reduce fluid flow to your rear calipers effectively reducing your overall braking performance.

A pressure regulator and a proportioning valve are two completely different things.

That's very inaccurate.
Braking systems don't' rely on flow, they rely on pressure. Once the pads are in contact with the rotors there is almost no fluid flow.
A good proportioning valve allows normal braking pressure up to the adjustment level, then reduces the rate at which the pressure increases.
If done properly it is every bit as effective as the factory regulator, maybe better.
The issue is that adjusting to the correct setting by doing brake tests on the road may be dangerous.

In fact, a poorly adjusted stock regulator will reduce overall braking performance even more than a modern proportining valve, because it takes a considerable amount of brake fluid to move the piston inside the regulator, allowing the pedal to continue moving with little increase in braking force.
In my experience, a large proportion of the stock regulators out there need substantial adjustment at this point to be effective.

I disagree.

I'm going to jump in Chris's passenger seat and ride shotgun on this one.

Flow and pressure are two different aspects of the system. Everything upstream of the regulator or proportioning valve relies on normal pressure dictated by the design. When the adjustment level is dialed in say down from the maximum a reduction in working pressure occurs. Even if upstream pressure is increased down stream pressure is regulated proportionally by its limiting orifice .

In the oem regulator downstream pressure is regulated by the same and piston movement in its bore resisted by spring tension. Here the upstream flow is changed in pedal travel where as the downstream pressure and flow is still limited by the lines and caliper pistons/ bore.

Lets throw a cog in for good measure. The oem master cylinder utilizes a dual circuit front and rear inside a common bore. If you disassemble a master cylinder you will quickly realize one circuit compresses against the other in its travel. What most people don't understand is the dual circuit masters actually create a bias within themselves. One circuit compresses less. Simply a conventional dual circuit inline master cylinder will not give you 100% equal pressure or volume out of both circuits throughout its travel.. That's why we have double master cylinder pedal options with bias adjustment. Yeah -I know its a tangent but so are pressure curves in closed system fluid dynamics..


Love you both. Happy holidays Gentlemen!

You boys have fun... I agree on the pressure aspect BTW. Just a choice of wording.

Don't know what the argument is about. It seems to me we're wanting to limit pressure (and flow) to the rear brakes to more closely follow the ideal braking force distribution (The attachment below helps to see why this limiting is a good thing). If we didn't do this limiting to the rear, the rear brakes would lock before the front brakes under heavy braking. And doesn't the Tilton valve offer the same characteristic as the Bosch valve with an appropriate adjustment, from looking at the pressure curves of both? It looks the same to me.

Anyway, I got to thinking about GeorgeRud's comment about the high point of the brake system, and so I made some measurements. Not including the master fluid reservoir, the high point in the hydraulic system is the T-fitting coming out of the pressure regulator valve. This point is a good 3 inches higher than the master cylinder or the top of the loop of hardline attaching to the rear caliper, which itself is another inch or so higher than the rear caliper bleed valve. That T-fitting is also higher than the hard line from the master cylinder to the valve. So, even though the master reservoir at the front of the car is higher than that T-fitting, air can still get trapped in that T-fitting because fluid comes to that point from a line that is lower.

So is air getting stuck at that high point, and that's what causes the spongy pedal? Maybe that's why people are saying to crack the line there to try to bleed that part of the system. Hmm, guess I'll think about how to put a bleed valve there to test the idea.

Click to view attachment

Well, in case anybody is following this thread ...

I added a T-fitting with a bleed valve right next to the tee fitting that splits the brakes to right and left rear. This second T-fitting is at the highest point in the brake system after the line from the master cylinder comes out of the back of the car, and it is at the same height as the T-fitting that comes out of the top of the original proportioning valve, which I had removed as noted in a previous post.

To recap, after removing that valve and replacing it with just a T-fitting, the brake pedal was a lot less spongy. I thought I might have solved the spongy pedal problem but I was bothered by the idea of air possibly being stuck at that high point at the firewall before the lines go right and left to each brake caliper.

So I put this second T-fitting with a bleeder valve at that high point. After bleeding the brakes the normal way, using only the bleeders on the calipers, I found as before that the pedal felt pretty good. But then I opened the new bleed valve on the firewall, and got several inches of air in the fluid line before getting solid fluid. So air was indeed getting trapped there.

After bleeding that air out, the brake pedal now feels excellent and solid.

Then I plumbed in the new Tilton valve so I have some brake proportioning. I made a temporary bracket out of sheet metal to keep things in place while I design a nice aluminum bracket to hold the valve and the T-fittings for the long term. Rebleeding as before retained the excellent pedal feel.

So the car is back up and running, with great-feeling brakes. Next is to figure out what's wrong with my original Bosch valve and why it wouldn't bleed.

It's clear at least that air gets trapped on the output side of that valve. Some people suggest cracking the lines at the output as a way of bleeding out that air, but that's not easy to do if my stock-configured car is any indication. It's very difficult to get a wrench on these fittings with the Bosch valve installed, hard to open and then close them in a timely manner like you can with a bleed valve that you have great access to. Cracking those lines would certainly get brake fluid all over the paint on the firewall, which I want to avoid on my freshly-painted car.

Maybe there's something wrong with my Bosch valve and that's why I couldn't bleed the brake system with it installed. It's been suggested that many of these valves don't work after 40 years. Maybe that's true.

So next on the agenda, after making a permanent bracket for the Tilton valve, is to explore why the Bosch valve doesn't work as expected. I'd really like to get that valve back in the car if I can.

I would think your next move is..... if you want to drive the car.....is to set the correct curve so the car doesn't spin on a hard stop if you keep the Tilton valve.

Yes, true. I set the valve according to the charts I got from Tilton and the diagram in the factory repair manual showing the Bosch valve characteristic. I tried to get it close based on those data. As soon as the weather warms up I'll go out and find an empty parking lot at night and make sure that it works as expected. I can reach the valve from outside the car so I can make any small adjustment that's needed.

Which model tilton did you get? Did it fit easily with no mods or is this a radical pedal modification? Pics?

Don't need to make a mess on the floor. Partially fill a small glass jar with old brake fluid and connect a hose over the bleeder fitting down into the jar. Baby food jars are ideal. Crack the bleeder valve as calling for a pump and watch bubbles exit the hose as pump is applied. then shut bleeder valve and call for mate to let up. Repeat till no more bubbles, then go to next bleeder.

@barefoot

I think Mark was referring to cracking the hard line connection, not opening a bleeder valve. There's really no way to crack a hard line under pressure without making a mess everywhere.

@ stugray

I used the Tilton 90-2003. You can see it here

Tilton Prop Valve

and you can see the document showing the performance curves in a previous post in this thread.

"Did it fit easily with no mods or is this a radical pedal modification?"

Somewhere in between. I guess it depends on your level of skill. When I was learning the trade, mechanics often made their own parts, but these days it seems most aren't inclined to do a job unless it's an easy bolt-on deal. This is definitely not a bolt-on deal.

I had to make several hard lines. By the way I used Eastwood's great line of brake tools to do the job. They make a brake flaring tool that makes this task much easier, and is better than any other tool I've seen for this job. You can see it here.

Eastwood Flare Tool

I also had to make a bracket to hold the valve and T-fittings. I did this fast and nasty, since I was just trying to prove to myself the idea would work. It took me about an hour and it looks very ugly.

I suppose I should have taken some pics, but as I said, I was in a rush to get it in and see if it works. Now that I know it works I will take some time to make a nice bracket, and I'll post pics as I do that.

I have re-read your posts and I have a few questions:

1. In one of the Bosch article posted, they (Bosch) make a distinction for the pressure proportioning valves for a given design. Static and Dynamic. Which Tilton valve did you install?

2. The output curve for the Tilton valve you installed...if I am understanding your post, looks linear. Doesn't that indicate that it is a static proportioning valve?

3. Are you planning to use the stock master cylinder and drive this car on the street?

Have you had the Bosch unit in the car since you got the air out of the top tee?
I would reinstall it and see if you can get the pedal to come up.

@ 914 teener

The difference between a static valve and a dynamic valve is:
* Static valve regulates output pressure as a function of input pressure only.
* Dynamic valve regulates output pressure as a function of input pressure AND a measure of vehicle load or rate of acceleration.

I haven't seen any aftermarket valves for sale that are dynamic regulating valves. The last time I saw such a valve was on a Fiat 124 I once had. That valve worked like the one shown in the Bosch document: there was a lever arm attached from the valve to the axle which moved up or down depending on the pitch angle of the car. That was Fiat's way of measuring load on the rear axle. That was a great car, wish I still had it.

So to answer your first and second questions: The Tilton valves are all static valves as far as I know. Certainly the one that I bought is a static valve.

Also, the Bosch valve that came on the car originally is a static valve.

And to answer your third question: Yes I use the stock 914-6 master cylinder and stock 914-6 calipers front and rear. I do drive the car on the street. I put maybe 2000 or 3000 miles a year on the car at most.

@ Bob L.

That's a good idea. I may do that if I can't figure out a way to test the valve off the car. I'm going to explore the idea of building a hydraulic test bench with which I can apply measured loads to the Bosch valve, and recreate the curves in the Factory Service Manual. This shouldn't be too hard. I'll need a pressure source (hydraulic pump capable of creating up to 2000 psi), a regulator, some pressure transducers, a couple of flow meters and a flow control valve. I haven't looked into it yet, but those things should be readily available.

The problem with re-installing the Bosch valve is it takes up a lot of space in that area and doesn't leave much room for anything else. The Tilton valve is tiny by comparison, and leaves room for the second T-fitting with easy access for bleeding.

But I do want to get the Bosch valve back in the car if I can, if for no other reason than keeping originality.

Ok....so if it is static then the output will be linear. Correct? The output is linear.

Doesn't the output on the valve want to be non-linear for driving on the street....meaning that in certain situations.....say a panic stop... you want a nonlinear output especially on the rears on a 914...otherwise the brakes would lock up...not good.

That is what I would think as dynamic output. Varying in degree to input.


* Dynamic valve regulates output pressure as a function of input pressure AND a measure of vehicle load or rate of acceleration.

But.......the engineers weren't concerned about the load...they were concerned about the polar moment and the rears locking and losing control as Newtons laws started to do their thing.

So if the valve you installed doesn't do the above...that could be a problem if you car is set up in a stock configuration. I would think?

@ 914 teener

I'm not sure how you mean to use the word linear here. None of these valves makes a linear relationship of output to input pressure. For instance, if we look at the Bosch valve in the following graph (I'll call it Figure 1)

Click to view attachment

we can see that the output pressure has a kink at the threshold pressure, after which the output pressure increase is reduced for each increment of input pressure. Compare the resulting pressure curve to the linear relationship shown for reference, which just keeps going up at a constant rate after the threshold pressure is reached. So the Bosch valve is NOT linear, and in fact all of these proportioning valves create a non-linear relationship of front-to-rear brake force distribution.

Actually, the engineers (of which I am one, by the way) are entirely concerned about load, since the ability of the tire to generate lateral (cornering) and longitudinal (braking) force is reduced with decreasing load, and load decreases on the rear tires as weight transfers to the front tires under braking. Thus, the ability of the rear tires to generate cornering and braking force is reduced during braking, and that is what can cause a spin to occur. The 914's lower moment of inertia in yaw does complicate matters somewhat, but all cars suffer from the problem of optimum or ideal front-to-rear braking force distribution, not just the 914. That's why most performance-oriented European cars, not just the 914, came with these proportioning valves before the advent of ABS.

The later versions of these proportioning valves came with the Dynamic feature you are talking about, as a way to further optimize the rear brake performance. Let's be clear, the "Dynamic regulator valve" differs from the "Static regulator valve" in that the Dynamic regulator valve changes the threshold pressure as a function of the loading on the tire, while with the Static valve the threshold pressure is constant (unchanging). This Dynamic valve was adopted even on the Fiat 124 that I mentioned, even though it had a high yaw moment of inertia, because the ideal brake force distribution (the pressure you'd like to have going to the rear brakes if you could get exactly what you wanted) changes with load. Note also that the lowly Fiat 124 had the Dynamic valve while the performance-oriented 914 did not! Why would this be? Hint: the 124 had a huge trunk that you could load with heavy stuff, thus changing the load on the rear tires more dramatically....

To see how load affects the need for rear brake pressure, look at Figure 2 ...

Click to view attachment

and you can see that the rear brakes can take a higher pressure (more braking force) at a higher load (max load) than at a lower load (min load). But the Bosch valve shown in Fig 1 is a Static valve (the threshold pressure does not change). The Bosch valve doesn't know what the load on the tires is, so its curve must necessarily be more of a compromise than if it were a Dynamic valve. Translation: the brake force curve will be closer to the "Min load" line so that the unwitting driver won't get in to trouble in conditions of high load transfer. This results in the compromise of the rear brakes being underutilized most of the time. You find this compromise on nearly all cars built before the advent of ABS. The engineers of that day tried to deal with this issue as best they could by introducing the non-linear force distribution curve with the kink in the curve at the pressure threshold point. The ability also to change from the max load case to the min load case with the Dynamic control feature came along a little later. Relatively few cars were equipped with dynamic regulator valves, mostly just the performance oriented ones. The rest just dialed back the rear brakes way way back. And then, ABS came along and changed everything.

Now look at Figure 3

Click to view attachment

to see how the Tilton valve can be adjusted to perform the same as the Bosch valve, with the same threshold point. There is, of course, no way to precisely adjust the Tilton valve to hit the desired threshold pressure exactly. I just found the midpoint of the adjustment range of the valve, then turned the adjustment knob a little bit more counter-clockwise. It's not exact but I think it's in the vicinity of being correct. As I said, I'll check it once the weather gets better.

This is a complicated topic and I'm sure I have already bored most of you to death. I apologize for the length of the post. But for those who might be having the same problem I'm having with the Bosch valve, hopefully you find this info useful.

DamonsCarrera - I realize this is an old post, but I'm having the same issues exactly, and if you're still out there, I'm dying to know how this all worked out. Any photos you can share? Thanks!