Need help-power booster brake diagnosis

mo-mon · mo-mon's Garage

After confirming that my brake pedal doesn't go down slightly when starting the engine, I checked vacuum by pulling out the vacuum hose in front-beside the power booster- w/ engine running......got good vacuum

Connected the vacuum hose back, pumped the brakes & started the car. Brake pedal goes down very little. Brakes are good but not sure if I am getting assist.

What should I do next? Pull out the engine side booster hose assembly & check for leaks or is the problem in the master cylinder/power booster?

Need your help gentlemen. Thanks in advance.

cashman · 11-01-2014, 05:05 AM

Did you pump the brakes the first time you started the car? Pretty sure you need to do that. You mentioned you pumped the brakes the second time and the peddal dropped. That's what its supposed to do.

bkreigsr · 05:46 AM

Not sure from your post what car you're driving (??) but I feel very little 'assist' in my 85 - I think that's how the factory designed the system. (read old school)
A good test for a vacuum leak is hold your foot on the pedal while stopped (motor running), and if it slowly sinks all the way to the floor - you have a vacuum leak, or bad seals in the master cylinder.
Depending on what year car (??) you have, if you are satisfied that the 'brakes are good', and assuming that the car stops fine, you probably don't have a problem.
Others more knowledgeable will surely chime in.
Bill K

mo-mon · mo-mon's Garage

Quote:

Originally Posted by bkreigsr

Not sure from your post what car you're driving (??) but I feel very little 'assist' in my 85 - I think that's how the factory designed the system. (read old school)
A good test for a vacuum leak is hold your foot on the pedal while stopped (motor running), and if it slowly sinks all the way to the floor - you have a vacuum leak, or bad seals in the master cylinder.
Depending on what year car (??) you have, if you are satisfied that the 'brakes are good', and assuming that the car stops fine, you probably don't have a problem.
Others more knowledgeable will surely chime in.
Bill K

I drive an '88 Carrera. Maybe you're right-expecting a "modern" car type of assist.

No sinking while stopped, so master cylinder is good.

Thanks.

luyckxglen · 01-28-2015, 03:37 AM

if it feels like ur pressing on a rock.. then something is wrong.. i have this feeling and allso have vacuum..
But a friend on his 80 SC has much better feeling on de brake.. so its not okay to just leave it like that

Number1_dog · 01-28-2015, 06:43 AM

I had a vacuum leak in my brake booster and had it rebuilt.(1979 911SC) and when I put it all back together I was surprised that it really was power brakes! My autocross co-driver said he perferred it before I fixed it.

Josh D · Josh D's Garage

For reference, I just went through this myself with my '80 SC.

The master cylinder had been leaking into the booster for awhile, however it still felt as if my brakes were working fine. Good pedal feel and no problem locking up the fronts. Pedal whould drop slightly with pressure on pedal during startup.

I finally replaced the master cylinder, but only drained and cleaned the original booster. Brakes felt about the same, maybe a slightly firmer pedal, during the test drive afterword.

A couple days later, the booster failed, and the pedal became really hard (no assist). I also had the accompanying vacuum leak.

Replaced booster with a good used one from a 3.2 Carrera. They are a bit bigger in diameter. Brakes are whole again, but pedal is much more firm than before. I prefer the feel I had before (softer pedal), but the braking is more progressive than it was, and the fronts don't lock up as easy. I think I'm getting more rear brake bias than I was before, which is a good thing.

After a week of driving, I'm used to the feel and wonder if this is how it's always supposed to feel. I need to do some hard braking, lockup tests to see exactly were my brakes are at the limit.

BTW, I now have very little pedal drop with pressure when I start the car.

luyckxglen · 01-28-2015, 12:20 PM

not sure if u really fixed it then i have a 3.2 carrera and wel you kinda feel it, it ur pedal is so hard and u really have to push all the was down to brake.. Ok u can brake because when i push it down hard all wheels just stop and my tires slip; But apperently with brake booster it should take much less effort..