Like a good mystery? 911 SC - help with brakes.

[JJK78-951]

Greetings everyone,

We have an interesting problem going on at our shop with the brakes on this SC. Pretty sure we have tried everything and are only down to replacing the booster but it just does not make sense - I am hoping the community can help us out on this one.

Has anyone had a booster go bad in a way where it would go to a soft pedal rather then a hard pedal? So say for instance you push the pedal firmly and it feels good, but if you keep steady pressure the pedal just goes to the floor, then you can pump it and it still stops but not really? Full (LONG sorry) story below...

911 SC inherited from father in law that came in for bad oil leaks due to broken head studs and brakes were all but completely locked down. We went ahead and replaced the master cylinder first then did a full rebuild on all 4 calipers. Split the halves, popped out the pistons, cleaned in a heated parts washer, cleaned passages with pipe cleaners etc. Now brakes worked pretty good but strange pedal feel like they were still bleeding down or had air stuck somewhere. After using line clamps we determined that the front brakes or front brake circuit on the master was the problem because with the lines clamped we had a nice hard pedal - since it was both sides we assumed bad master and replaced it again...

With the 2nd new master on the pedal felt better, good enough to drive so we fired it up and after a few pumps of feeling ok and pulling it out, then the pedal started to get mushy again, like the booster was over boosting or air in the system. Turn the car off and the pedal feels better, but not great. Several bleeds and checking lines etc. later, we decided to split the calipers again on the car just to be sure the fluid passages were clear - they were. So we did more on car testing with line locks and found this time that the fronts were holding pressure just fine, but the rears seemed to be the issue like a bad circuit on the cylinder. Didn't make sense but we went ahead and got a different brand master cylinder (#3) from a different supplier to try and avoid any possible manufacturer defects. Still no dice.

Problem still seems to be with the rears as I ran the car on the lift and initially the brakes will stall the motor if you stab them, but if you take them to threshold where they start to slow and apply steady pressure the pedal just goes to the floor and will not stop the rear brakes. So even though the brakes hoses were in pretty new condition and not showing any signs of flex etc. we went ahead and replaced those just to be sure, nope. Pulled the calipers completely apart again today, pistons out etc. fully cleaned again - still have same issue.

So here we are, the only things not replaced, rebuilt, checked etc. are the hard brake lines and the brake booster... We are pretty much ruling out the hard lines and just don't see that as a possibility. That only leaves the brake booster as we have triple checked everything in the hydraulic portion on the system, but the booster seems like it would show signs of failure and/or the failure result would be a hard pedal. Holds vacuum pressure just fine.

Final question/request - anyone in the Atlanta area have a brake booster from an 78-89 911 we can test with or buy from you??

Sorry for the long post and thanks in advance if you read the entire thing!
John

[prschmn]

Let me offer my sympathies for your plight. But you seem to have tenacity-you're needing it
for sure.
Not sure I can offer a lot here but---
Since in the end the booster has a mechanical connection a sinking pedal pretty much
has to be saying there's a leak--a very small one.
Are you using quality rebuild kits or that Centric stuff?
I've used it with mixed results myself.

[uwanna]

I would bet it's a master cylinder issue, even though you have used several!
Read this thread about the poor quality MCs being sold now and the symptoms they cause. Sounds a lot like what you are going through!
ATE master cylinder issue?

[JJK78-951]

Originally Posted by prschmn

Let me offer my sympathies for your plight. But you seem to have tenacity-you're needing it
for sure.
Not sure I can offer a lot here but---
Since in the end the booster has a mechanical connection a sinking pedal pretty much
has to be saying there's a leak--a very small one.
Are you using quality rebuild kits or that Centric stuff?
I've used it with mixed results myself.

Thanks - that is the only thing I can think of is a very small leak in the booster somehow causing this effect. We only use OEM parts, first 2 cylinders were ATE, #3 that is on the car now is an Erling I believe? No Centric stuff.
I appreciate the reply!
J

Originally Posted by uwanna

I would bet it's a master cylinder issue, even though you have used several!
Read this thread about the poor quality MCs being sold now and the symptoms they cause. Sounds a lot like what you are going through!
ATE master cylinder issue?

Yeah I hate to think that but really seems like it is a bad master - makes more sense then the booster anyway but after 3 you would think 1 would work properly! Thanks to the thread link and your response!
J

[IMONBOOST]

Do you have a proportioning valve? It could be leaking or having problems, just throwing this out there.

[JJK78-951]

No proportioning valve on this car - 82 SC. Thanks for the suggestion!!

After reading through that other thread it does not sound like anyone found a solution. This sucks. Seems like the master but doesn't sound like anyone even suspected the booster so I'll just chuck that idea for now... Anyone had luck with the URO parts one? I'm willing to try anything at this point.

THANKS!
J

[uwanna]

Originally Posted by IMONBOOST

Do you have a proportioning valve? It could be leaking or having problems, just throwing this out there.

There is no proportioning valve on an SC, that started with the '84 Carrera.

[ossiblue]

A bad check valve on the brake booster could account for the problem. The check valve prevents air from entering the booster--it only allows air to exit (vacuum). A faulty valve can cause, among other things, an excessively hard brake pedal or the opposite, a spongy feel to the pedal. The fault can eventually allow air to enter the master cylinder causing loss of brake power to the circuits of the hydraulic system. There have been a few threads on this forum which describe hydraulic fluid being drawn into the intake via the brake booster vacuum connection, so the relationship between the booster and the hydraulic fluid in the MC is there.

From reading your posts, you have eliminated all possible suspects, save the booster, either through replacement or R/R. IMO, the booster is definitely something to look into. If the check valve on the 911 is not easily accessible, the fault could still be with the booster as the symptoms you describe are consistent with air entering the system and into the MC.

Here's a picture of the possible suspect-the part with the white disc, connected to the hose, located at the right of the photo:

[JJK78-951]

Originally Posted by ossiblue

A bad check valve on the brake booster could account for the problem. The check valve prevents air from entering the booster--it only allows air to exit (vacuum). A faulty valve can cause, among other things, an excessively hard brake pedal or the opposite, a spongy feel to the pedal. The fault can eventually allow air to enter the master cylinder causing loss of brake power to the circuits of the hydraulic system. There have been a few threads on this forum which describe hydraulic fluid being drawn into the intake via the brake booster vacuum connection, so the relationship between the booster and the hydraulic fluid in the MC is there.

From reading your posts, you have eliminated all possible suspects, save the booster, either through replacement or R/R. IMO, the booster is definitely something to look into. If the check valve on the 911 is not easily accessible, the fault could still be with the booster as the symptoms you describe are consistent with air entering the system and into the MC.

Here's a picture of the possible suspect-the part with the white disc, connected to the hose, located at the right of the photo:

Thanks! That is definitely something to look into. I'll get with the guys on Monday and see if that could be an issue, I appreciate the response!
J

[blue911rsr]

Are your bleeder screws at the top have seen calipers installed on wrong side with bleeders at the bottom will not work that way

[uwanna]

One thing that can cause a problem with the vacuum booster is the rear seal on the MC (facing the booster) can fail, causing brake fluid to run into the booster body and over a period of time ruining the rubber diaphram in the booster. When you take the MC off the booster, turn the booster upside down and see if brake fluid runs out, if so the booster is probably shot.
Had that problem on my '80SC.

[manbridge 74]

I would bypass the booster (back at engine if possible) to confirm that problem remains. Car should stop consistently albeit with an increase in pedal effort required.

[JJK78-951]

I definitely appreciate all the ideas guys! We will test more on Monday and i'll report back~
THANKS!
J