1989porsche928

Hello everyone. It has been a while since I had time to drive my car. I noticed today I have to press the brake in all the way for the car to stop. The car has brake fluid inside it. Do you think the brake booster is bad? I am confident I need new brake pads and rotors, just want to make sure I get my brakes fixed if I am going to take everything apart again.

FredR

The brake booster can be "tested" by starting the car with your foot on the brake pedal. When you start the car you should feel the pedal go soft as the brake booster kicks in with the build up of vacuum. A bit empirical but if the booster has gone you will not feel this action. You need to ensure you are pulling a good vacuum at idle and check the vacuum hose from the inlet manifold to the booster.

For the brakes you need to ensure that none of the pistons have seized- the effective way to achieve this is with the car on jack stands and check each pads to ensure it is firmly engaging the disc. Before each test at each wheel pry the pads slightly apart and check to ensure both sides close up when the brake is depressed- better with two people but I did this myself.

Beyond that make sure you have at least 2mm of pad thickness on each pad. I recently had a judder-more noticeable when braking from speed. Had no clue what it was initially. Road tests suggested it more likely to be in the rear and there was no pulsing when the pedal was slightly depressed. I removed the pads and cleaned them up a bit but still a problem. I noticed that the rear discs were not polishing with use so removed the discs and gave them a good clean. I suspect the problem was created by washing the car and not immediately drying off the discs. I rubbed the braking surfaces with a sheet of wet and dry [medium grade] until I felt the residue was off the disc face.

Reinstalled everything - I did not have new dampers for the rears so I cleaned the existing dampers and and used some blue silicon RTV to help seat them on the back of the pads.

Everything now seems to work fine. If elements of your system are worn out then simply replace them.

Rgds

Fred

WallyP

Your description isn't too clear - we can't tell whether you mean that the pedal goes down too far, but operates normally otherwise; or whether you mean that you have to press too hard on the pedal, but the pedal position is normal.

One quick check can help determine what the problem is.
Crank the engine, let it run for a moment.
Press the brake pedal with normal pressure, noting how firm it feels, and how far down it goes.
Shut the engine off. Press and fully release the brake pedal several times.
Wait a moment, with your foot off the pedal.
Press the pedal, noting how firm it feels, and how far down it goes.
If the pedal is high and hard, the master cylinder is possibly OK. If the pedal is soft, there is probably air in the system, or the master may be faulty, or a brake line may be soft and swelling.
Release the pedal, then very slowly and gradually press the pedal, very slowly increasing the pressure, while noting if the pedal slowly sinks under your foot.
If the pedal slowly sinks under pressure, the master cylinder is old and worn. The rubber cups are not flaring to fully seal in the bore. Replacement is the best option. The usual term for this problem is "bypassing".
Release the pedal, press firmly and hold the pressure, and start the engine. If you have a five-speed, make certain that the gearshift is in neutral! Note what the pedal does as the engine starts and runs.
If the pedal remains high and firm, the vacuum booster is not functioning. The pedal should sink and get softer.

1989porsche928

The brake pedal easily goes to the floor. When it is all the way down it barley slows down and stops the car. So if someone slammed on their brakes I could not stop in time.

drwhosc

Just wen through a similar situation. Master cylinder was going going gone. If your pedal is going to the floor, prob you master cylinder. if pedal is hard to press, no bake boost.

When I relapsed my fluid, it was dark and nasty. I also rebuilt my calipers. They are stout, and were actually fine. A little tricky to do, but in retrospect, I do not think it was necessary.

First thing to do is change the fluid, make sure there is no air. May improve your situation, but kind of doubt it. you prob will have to replace the master cylinder, and given the age of the car, prob not a bad thing to do.

Also now is a great time to check your E-Brake. You might need it if you continue to drive....

FredR

This now reads somewhat different to what you posted initially- no big deal but as Wally identified, it does help those who want to help you diagnose the problem and identify the way forward. Just follow his excellent all in diagnostic recommendations.

For starters an S4 that cannot stop must not be driven on the roads. Your brake discs should have a polished sheen on them and on both sides- the outboard side is easy to see, the inboard side more or less impossible to see without removing the calipers and discs. All things being equal if one side is clean the other will be. Take a look at your discs and advise what you see if there is anything of note

If I was to hazard a guess at the moment I would be deeply suspicious of the master cylinder. Replacing the brake fluid by bleeding the brakes may produce some results but if the problem suddenly appeared for no apparent reason then to my thinking that suggests a master cylinder issue. Have you experienced any issues with brake fluid level?

Rgds

Fred

dr bob

Does pumping the pedal cause the 'pressure point' to rise up some in the pedal stroke? If so, that's a common symptom of air in the hydraulic system.

My car was showing a little of that, with a low pressure point that rose up in the pedal stroke if I pumped the pedal a couple times. I had some air trapped in the fluid circuits, particularly it had found a home in the ABS pump. Car was due for a major brake system service anyway, so it all came apart for that, and some careful power bleeding when all the parts were connected again.

MainePorsche

Yes, good careful bleeding is crucial to the function of the system. I can't wait to do it again when I put my new rotors and pads on after the winter hibernation.

OTR18WHEELER

Sound like you have air in the system.
The rubber grommets beneath the reservoir may need replaced, and/or the master cylinder may be bad.

MainePorsche

+1

dr bob

FWIW, I ended up replacing brake hoses, MC reservoir grommets, rebuilt the calipers, plus new pads and rotors, all as part of a full system overhaul at about 110k miles.

That effort also included a seriously extensive brake bleeding episode, some of which was induced by having fugitive air pockets in the ABS unit. Symptom was lower pedal pressure point that would pump up, plus a less-than-firm pedal even after the pump-up. Getting that air out took a combination of vacuum bleeding and a combination of forward then reverse pressure flushing each section of the hydraulic system, starting at the calipers and working back towards the ABS unit then the master cylinder.


The booster check is easy-- run then stop the engine, and try two to three presses on the pedal. You should get two fully-assisted presses on the pedal. Then the third will start to get harder to push, The fourth will be hard to push down, as you have run out of vacuum in the booster can. Also, when a booster diaphragm fails it will draw brake fluid out the rear into the front of the booster. You'd notice the loss of fluid from the master cylinder fluid reservoir. The other failure point is the check valve, which would demonstrate a hard pedal if you pressed on it while the engine is under high load (accelerating).