For a long time I've been thinking the rear brake pressure limiting valve is a source of weak braking in many 914s, and I want to sort out how to make adjustments which will create a real improvement without increasing the likelihood of rear lockup.

Yesterday I completely disassembled a unit for the first time.
It's function is very simple.
Now I understand how it functions as a pressure limiting valve as opposed to the proportioning valves commonly installed in race cars.

There are two aspects to my experiment.
First, vary the pressure at which the unit activates.
Second, turn it into a true proportioning valve.

I will conduct tests using two methods.
One - bench testing with pressure gauges to measure and graph the difference between line pressure and output pressure.
Two - drive the car to feel any differences.
The second method involves some danger if the rear force is too high and lockup occurs, so I need to find a safe location to conduct these tests.

In order to vary the pressure, since the set screw is nearly all the way in from the factory, I have to shim the large spring inside the housing to increase the potential force that can be applied to the valve.
Then I can use the allen screw at the end of the housing to alter the static spring length.
The shims can be installed without opening the hydraulic portion of the valve.

Turning the valve into a proportioning valve appears to be a simple procedure.
Removing the large hex cap from the other end exposes the valve plunger.
Once the plunger is removed from the valve body I can remove the brass hex piece which retains a spring loaded shutoff valve.
After removing the shutoff valve pieces and reassembling the unit, I believe it will act just like the racing prop valves.
Ie, at the pressure where the big spring starts moving, the rear brake pressure will begin to increase at a slower rate than the front brake pressure.
The spring rate determines the differential rate, so I might even be able to find other springs to install to alter the differential rate as well as the pressure at which the valve begins to operate.

Do you have, and have you reviewed the factory manual info on this valve Chris? It gives some good tech detail.

Yes, I'm going over it now.
The description indicates that the limiting valve does act the same as a racing prop valve, with the check valve opening and closing seamlessly as brake pressure increases.
I'll have to noodle over that a bit more, since what I feel during heavy braking is that the spring continues to collapse and fluid displacement occurs, allowing the pedal to travel too far. I need to know if that check valve is really functioning as the manual states, or if once closed it stays closed until braking pressure is reduced or released.

Very interesting.. I'll keep and eye on this one...

Me too. Very timely, because I just posted a question about replacing mine with a T-fitting! Thanks for the doing the legwork.

Thanks for doing stuff like this Chris.

This is going to be really interesting - thanks!

John

Cool

This is very interesting.

I have to ask though, why not just replace this stock valve with a readily available adjustable prop valve?? Is it against your class regs??

This isn't for racing.
As long as I've owned a 914, this component has been a source of trouble and mystery.
Right now, I don't think the one in my DD functions as the factory manual describes.
So I'm trying to determine if there's an effective way to update the stock system without resorting to substitution of components.

I put a wilwood lever type in my car and disassembled my original a while back. i think the parts are all bagged up or sitting in a box somewhere. If you need it for some spare parts shoot me PM (sometimes i have trouble with topic reply notifications). I'll gladly donate some 914 organs to research!

I see. .. Cool. I'm a "run whacha brung" kinda guy so this interests me. . .

Any chance you can post pics of the tech documents?

I am visualizing that you would need a check valve that works the opposite of the ones like on an air compressor.
This one must remain open until the brake pressure climbs to the preset value, THEN close and stay that way as long as the pressure remains above the set-point.

However, if that was the end of the story, you would not be able to press the pedal any further to increase the pressure to the fronts.
That is because of our in-line master cylinder configuration. Close a hard valve on either one and both will stop moving.

So... there must be an accumulator inside that allows the volume to continue to increase without raising the pressure on the other side of the check-valve.

So there must be a valve (the adjustable one) and a second spring/piston combo...

I would love to hear more about what you find inside.

Stu


edit: Unless the two linear pistons in our master cylinders are connected by a very stiff spring instead of rigid connection....
then one could stop (the rear) while the other (front) keeps moving...

This is really interesting, especially since am in the process of completing overhauling the brake system on my restoration.

Can you post a copy of that section you are reading from?

No time to scan it right now.
Factory Repair Manual
Chapter 6 - Brakes
Pages 0.1-4/1,2

Here it is in .JPG form:

Click to view attachment

I think I originally mis-read your comment (or you changed what you wrote??)



-or now-



I think it's pretty cool that you are digging in to this as it will "HOPEFULLY" change the myth, once and for all in the 914 communities that this is a "Proportioning Valve". I read Alfa forums at times and it's amazing the polarity with the 914 guys on this subject. The important factory manual text is the very last sentence and the words "regulating range". That valve opens and closes when it is in the "regulating range" meaning, once above 525psi this valve does it's thing. Below that... it simply passes fluid like a "T".

I do have major concerns I'll voice though.

1. This is a safety device meant to prevent rear wheel lock in the case of a panic stop. As Chris has now pointed out, it is not like a proportioning valve commonly sold for race applications. Chris, owning your own biz, you may want to add a disclaimer regarding safety and messing with these type components. Renegade still recommends "T's" in their printed literature for 914's and I'm shocked. Any postmortem litigator would have a field day with that. As a respected member here and all-around smart guy, keep in mind a lot of these people will follow you off a cliff, especially if there is some sort of perceived "performance enhancement". 99% of the readers here should NOT TOUCH THIS (that still leaves 77 people I'd be worried about). Dr. Evil, please post pictures of your accident.

2. In "turn it into a true proportioning valve" I would again add that 99% of you have the "opposite" need for such a valve in a 914. As Chris explains... "After removing the shutoff valve pieces and reassembling the unit, I believe it will act just like the racing prop valves. Ie, at the pressure where the big spring starts moving, the rear brake pressure will begin to increase at a slower rate than the front brake pressure."

Allow me to explain:

The MOST DIFFICULT THING TO DO IN A 914 IS TO FIND A LARGER REAR BRAKE SOLUTION. When I say this, I mean a true bolt on solution. The largest you can go and bolt it on is the uber-rare 914-6 rear caliper. That means; no grinding, no welding, no aftermarket cable parts, no 911 bits etc. Those calipers are easily over $1,000 per pair. Compare that to 911 handbrake solutions and various other "money is no object" calipers that can be made to fit... and you are "50:50" on dollars spent. This is more than most have invested in the purchase of their 914 which is why few do it and MANY will be looking for any type of performance enhancement with drool coming out of the corner of their mouth.

The point? The only need for a true proportioning valve in 99% of the vehicles out there would be to "limit" the rear brake pressure. Again "...the rear brake pressure will begin to increase at a slower rate than the front brake pressure.". The only need for this would be if you bias was now set heavier toward the rear of the car. Quite simply... if you put larger calipers on the back. As mentioned above, that's one of the most difficult things to do on a 914 while still having a ) a handbrake cable and b ) money left in your wallet.

The bias of your brake system was set by the factory engineers. Check the piston sizes on all Porsche vehicles from the 1965 356C on and you will find a fairly constant ratio. None of them have a proportioning valve. Putting a proportioning valve on a 914 can only limit your rear braking ability. This would be the opposite of a brake enhancement for 99% of you. If you're driving a 908 with S-Calipers on all four corners... that valve would come in handy. If you mistakenly put early front 914 calipers on the rear and you don't care about a handbrake... that valve may come in handy.

Most 914 brake "upgrades" have included larger bolt on "front" brake solutions (BMW etc) with no regard for the rear bias. Hence negating the need for a proportioning valve in a 914 (unless you can figure out how to make it work backwards).

Carry on. I'm excited you're digging into this and more excited to see what you come up with.

IMPORTANT - This is a safety device gang. "Please" think 2-3-4-5-600 times before attempting to adjust yours, replace it with a T or modify your brake system at all. This is basically one of the very first "Anti-Lock" brake devices on the market. It will shut your rear caliper off in a panic stop and bring them back into the system as pressure equalizes in that chamber. Think... think... think... Good thing? Bad thing? Good thing? Bad thing?

Stu's pretty close in his reply.

I agree that its a good thing.
The last thing I ever want to occur is to have the rears lock up first in a panic stop.
In my race car, I can immediately move the bias forward just a little if I feel the rear end dancing when I'm threshhold braking, and therefore avoid that scary feeling that comes when the car rotates unexpectedly.
Rear lockup happens easily in older pickup trucks, but they don't have much weight at the rear to make 'em come around on you.

Eric, where you quoted me, the first quote was written after looking at the factory manual, while the second quote was written earlier, after disassembly and inspection - but before reading the manual. Ie., what I understood after studying the assembly didn't match what I read in the manual.

Part of my investigation is to determine if the valve actually works as the factory states, and if they may only need adjustment at this age to bring them back to factory spec. Its well known that the stock rear suspension springs lost rate over time, so maybe the control valve spring does too.
The other part is to determine if some level of modification will enhance their function without compromising safety.

There is a check valve in the primary circuit which apparently works as you describe, and there's a piston accumulator pushing against a large spring in the secondary chamber. The secondary chamber stays dry except for minor seepage over time.
However the factory description indicates the check valve function is more complex than merely not allowing further pressure increases to the rear brakes.

Again, I think it's a good thing that someone like yourself who is respected in the community is shedding light on the "actual" function of this device.

Almost "everyone" call this a "Proportioning Valve" and because of the notorious difficulty in bleeding 914 brakes, it gets a bad rap.

Gang, PLEASE keep in mind that Chris is a very good race car driver and, his description of bias and the "dance" is spot on. I've been there many times in my 911 at the track. It is not a fun feeling. A 914 with "polar moment inertia" can be even more dangerous. Yes, we're blessed with a mid-engine but, if a spin happens, you are now akin to that age old child's play toy... a "Top". This is why this valve is in a 1970-76 914 and "not" in a 1970-76 911.

Race car driving is like sex; most guys think they are really good at it... only a few really are. Most guys practice a lot and few do the deed regularly. Guys like Chris and Brant and Brittian could drive circles around most of us... in a Yugo. Chris probably knows more "intuitively" about bias than we do about driving in general.

Be careful when you mess with your brake bias. Especially if you are not driving professionally or competitively.

You have to read the "Operation" section to understand that it involves equalization of pressure in the chamber and on and off functionality. I believe there is a better write up, including graphs, in the Alfa manual (now, where did I save that file?)

I cannot download those pdf files.
I have tried from work & home, so I dont think it's a problem on my end.

Stu

Same here... that's why I posted the JPG.

I assembled my bench testing setup on Saturday.
The rig includes a M/C with a hand lever to actuate the pushrod, one caliper for each circuit, and a stock limiting valve in between the M/C and the rear caliper.
There are gauges in the rear circuit before and after the limiting valve to observe the differential pressure.
I inserted blocks into the calipers to keep the pistons from pushing out.
The outer casing of the pressure limiting valve has a hole drilled in it to observe any movement of the spring.

After bleeding the system thoroughly, I still wasn't able to create enough pressure in the circuit to actuate the limiting valve before bottoming out the M/C.
I've had this caliper test rig for a long time, only without the pressure limiting valve. Maybe the old 17mm M/C is bad, so I swapped in a better looking 19mm M/C from my parts collection.
I think this M/C has a bad seal too though, because I have to pump it up a couple of times before getting significant pressure buildup in the circuit.
The highest pressure I've been able create is about 1000 psi, only half of what I hoped to reach, but more than enough to actuate the limiting valve, which is supposedly calibrated to start functioning below 700 psi.
So above 700 psi the gauge after the valve should read less than the first gauge.
I can observe the spring moving, so I know the valve is being actuated, and I can hear it as well.
However, the observed pressures are equal on both sides of the valve.

These initial results are a bit confusing.
It appears that this limiting valve is not functioning as the factory manual describes.
It does start moving at approximately the correct input pressure, but doesn't seem to be reducing the output pressure.
According to the manual, the piston is supposed to move back and forth, opening and closing the check valve to allow a reduced rate of increase.
What I observe is that once the piston starts to move, it continues in the same direction as the pressure increases - which is the same thing I feel/hear happening in a vehicle with stock setup.
Except I thought this meant the output pressure wouldn't go above the actuation threshhold, with all increases absorbed by the piston displacement.

I did a seat-of-the-pants operational test as well on Monday.
One of the customer cars here is in for brake system diagnostics. The pedal felt a little soft in heavy braking, and the proportioning valve is indicated as the culprit. On the lift, with me in the cockpit testing the pedal feel, Ed tightened the adjustment screw by first one, then 2 turns (3mm). Then I went for a ride to do a brake test.
The pedal definitely feels a little stiffer at high braking pressures, although I can still feel the prop. valve actuating.
In fact, the brakes feel real good in this car now, where they were so-so before.
I was able to modulate the pedal to barely achieve front lockup in a hard stop from 60 mph, with no evidence of excess rear brakes.
The combination of tires, brake pads, etc. all influence the possibility of achieviing lockup.
With this car, and the change we made, I can get the front tires to howl on the pavement without ever fully locking. It was very controlled threshhold braking, about the best I've ever felt in a stock street 914.

I'll keep working on the bench test rig until I feel its functioning properly and producing valid data.
I really want to correlate my operational test results with bench test results so any vehicle adjustments are fully understood, and 100% safe.

Testing info from the factory manuals, including pressures.

A bit off topic but the illustration in the manual is a 6 unit. ( and displayed upside down )
I recall vaguely that 4cyl. and 6cyl units are plumbed differently and may work at different pressures.

They are the same except for the pressure setting where limiting starts.

I didn't look at the -6 section earlier, and didn't see a test procedure description in the -4 section.
I guess I'll move the upstream gauge to the front caliper circuit and re-test.

Doh ! and very interesting topic Chris.
Thank you for exploring this often misunderstood device and shedding light on it for better understanding. And better braking !

I believe there are three versions;

2 914-4 - Early and Late with different inlet configurations and,
1 914-6 with a larger voulme bore.

The differences are minor.
Early 4 and 6 differ only in the pressure setting.
The late -4 units (starting with '74my) have revised outlet plumbing, so they have one more drilled/tapped hole on the body of the unit.

Check with Mark Heard. I believe he had a NOS -6 version in a different size. I don't have a -6 here to compare.

I learn something new about 914s from time to time.

PET lists 3 different PNs for the "Brake Pressure Regulator"

91435506700 914/6

91435506500 914 1.7, 2.0 thru VIN 4742915751

91435506501 914 1.8, 2.0 from VIN 4742915752