Rear brake pressure regulator valve, Brake repair Options

[DamonsCarrera]

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?

[toolguy]

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. .

[DamonsCarrera]

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.

[nine9three]

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

[toolguy]

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.

[914_teener]

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+1

[Mark Henry]

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.

[ConeDodger]

Don't remove the valve. You will eventually come in contact with Eric Shea and he will poke your eyes out. (IMG:style_emoticons/default/blink.gif)

[walterolin]

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.

[gothspeed]

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+1

+2 .... (IMG:style_emoticons/default/agree.gif)

[r_towle]

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.

[DamonsCarrera]

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

[Eric_Shea]

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:

 Pressure_Regulator.pdf ( 83.82k ) Number of downloads: 504


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.

[DamonsCarrera]

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.

 Bosch_ISBN_0_8376_1391_4.pdf ( 1.92mb ) Number of downloads: 303


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. (IMG:style_emoticons/default/cheer.gif)

-Damon

[ChrisFoley]

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.

[Eric_Shea]

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

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.>>

when I take the valve out of the system I can bleed the brakes just fine.

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.

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.

[DamonsCarrera]

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?

[GeorgeRud]

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).

[DamonsCarrera]

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.

[DamonsCarrera]

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.

 Tilton_98_1261_Brake_Proportioning_Valve.pdf ( 410.95k ) Number of downloads: 596