mytrplseven

I had figured that since the rear brakes were a little cooler after road test the proportioning valve was doing its thing. but it's an easy replacement, I guess I'll get on it. I have to wait for a replacement aux water pump relay to show up anyway, I'll order another valve.

thom4782

Is it possible that the check valve is installed backwards? Probably a snowball's chance, but one never knows

mytrplseven

The check valve is new and installed as it should. When the new booster was installed, I did a "suck" check on it and used a new grommet to install it. I didn't bench bleed the master cylinder but with a hard pedal and the numerous times the system was bled manually I felt it was ok and I did put brake fluid into the connection and reservoir holes. I've ordered a new proportioning valve as I don't trust that I might not get a proper one from salvage (I read some recent forums about the different ratios on valves for different 944 models). That was insightful and timely as I might have not known this in advance and purchased one for the wrong model 944. The flex brake lines are new with braided outer coverings and I had to replace one hard line leading to the rear driver side caliper. The connection was so beat up that I couldn't get a wrench on the caliper fitting and had to resort to vice grips to remove it during the caliper rebuild.

thom4782

With everything you've done, it seems there are 3 possibilities left: a non-functioning booster, brake pads or rotors. I don't have any idea how to test it, but proving the booster is functioning properly might be the step.

mytrplseven

This is a brand new ATE booster. I applied 25 inches of vacuum to the input hose to the booster and it held the vacuum until the brake pedal was depressed. As I understand it, this is normal function. if there was vacuum available from the engine while it was running, there would be a constant supply to keep it up. I had thought about the pads as the rotors are in great shape and are Zimmerman cross-drilled units. The pads are Hawk performance pads and I don't believe they are ceramic but I'll have to check. They're not glazed and I dressed ;them before I re-installed them following brake caliper rebuild.

951TurboRS

It sounds like you still have air in the system. Are you pressure bleeding or pumping the pedal?

My bleed procedure is to first use a pressure bleeder to flush the system. If your only using the brake pedal it gives air a chance to float back up after every pump.

After pressure bleeding I remove the pressure bleeder and with two people do the pump and release method. Don’t use any of the one man bleeding gimmicks nothing beats a seconds set of hands. Be sure to use a fair amount of pressure on the pedal, the more pressure the better.

Finally if I’m still not happy I put the pressure bleeder back on with an empty reservoir (not the master cylinder reservoir just the bleeder reservoir) as you don’t want it to leak. Pressurise the bleeder to its maximum and let it sit overnight. This helps draw any trapped air back to the reservoir. Then repeat the process.

I have done this time and time again and it works.

mytrplseven

Now you have me confused. What is the bleeder reservoir? I can't picture putting a pressure bleeder on an empty fluid reservoir. I always connect my power bleeder to the top of the fluid reservoir. My procedures were the same as yours, i.e., flush with pressure bleeder, two man bleed (which resulted in solid pedal) and then pressure bled again to ensure the calipers were devoid of any air. Since then I've replaced the booster and did the whole mess once again. Now, under the suggestion that my rear brakes may not be performing, I've ordered a new proportioning valve. Looking forward to your clarification of the "bleeder reservoir".

Humboldtgrin

Yea I’ve never head of a “bleeder reservoir” myself either, unless your talking about the pressure bleeder tool itself.. Now from what I think I know the rear brakes are applied during the top brake pedal movement and then the front brakes are applied as the pedal is pushed down more. And the proportioning valve is to allow more pressure to the front brakes as the pedal is pushed harder. It aids in the handling of the car so the nose of the car doesn’t dramatically drop down when you push on the brakes. With the rear brakes applied first the rear of the car “sits down” first then the front of the car follows. If the rear brakes aren’t working then you have a hard pedal and end up half way thru an intersection. You know you need to check the rear caliper pistons to make sure they are not stuck/frozen. That would not be good.

MAGK944

Sorry dude but none of that makes sense unless you don’t have springs. Regardless if braking is applied to the front or the rear of the car the sprung weight will shift forwards, that’s just physics. The rear will never “sit” unless maybe you were driving backwards.

And during braking, because the weight shifts forward the front brakes are always bigger than the rear as they do more work.

Afaik the prop valve is simply a fixed bias valve which determines the hydraulic pressure going to the calipers.

jerome951

mytrplseven, I'm having a difficult time understanding the symptoms you are seeing. Are you experiencing...

1. brake pedal goes to the floor and stopping distance is increased, or
2. brake pedal is very hard to push and does not move much, and stopping distance is increased, or
3. pedal moves a few inches t and is firm, but brakes are weak; pushing harder doesn't change the stopping power

If #2 or 3, it's highly unlikely you have air in the system and bleeding won't help. I'd appreciate if you could clarify.

951TurboRS

Sorry for the confusion I’m talking about the pressure bleeder reservoir. The last thing you want is a fluid leak while it’s unattended, if I ever leave it pressurised overnight I make sure I’m just using the air pressure. The alternative is to simply apply the brake and lock it on quite hard and leave it overnight. Even just applying the brake and holding it for 5 or 10 minutes can do the job.

Charlie

I had the same problem with an 86 and 87 951. Never solved it so I read all I can about weak brakes.


Here is a link to a similar thread.

https://rennlist.com/forums/924-931-...ak-brakes.html

951TurboRS

I was about to mention brake pads, Charlie’s link above does as well.

I had some pads on my Cayenne that felt soft, they just didn’t bite with confidence at all. They started to work with a bit of heat but still had low bite. It’s a result of design regulations where a pad must be compliant (low friction coefficient) yet be able to pull the car up 2 or 3 time in row from its maximum speed without fading. On a big heavy Cayenne turbo that gets close to 300kmh it makes for a hard test to pass.

Because of this I always opt for an offroad non compliant pad with a higher friction coefficient. Don’t go to high though as you can end up with overly sensitive brakes. If you ever delete a brake booster a high friction pad is a must.

Droops83

The style of proportioning valve in Porsches of this era is referred to as a "brake pressure regulator" by Porsche, and each model had its own specific version with a different pressure rating. The 951 brake pressure regulator can easily be seen, it is screwed into the front of the brake master cylinder. As seen in the pic in the link below, it has "5 18" stampings, which means that at brake line pressures of between 5 and 18 bar, full hydraulic pressure is directed to the rear brakes. Above 18 bar, the rear brake line pressure is reduced in a linear fashion in proportion to braking pressure (hence the common name of the part), which helps to prevent rear brake lockup. The 911 3.2 Carrera used the same style of valve, but the rear weight bias meant that rear line pressure wasn't reduced until 33 bar of rear line pressure; the 930 version didn't reduce until 55 bar!

https://www.autohausaz.com/pn/PO-95135530501

The manually adjustable pressure regulating/proportioning valves as sold by Tilton, Wilwood, et al work the exact same way---the manual adjustment is simply setting the initial point at which the rear line pressure decreases. This link helps to further explain the function of both OE and aftermarket types:

https://tiltonracing.com/wp-content/...ning-Valve.pdf

MAGK944

Thanks for the technical theory, I use a Tilton version in my track car and never really thought about how it worked, I just know it does what I want it to do. The amount of knowledge on these forums still amazes me