Weak braking: cylinder or booster ?

[Shorty]

I've got a '81 european 924. I've got it for about a year, and have had a lot of problems with it, but (after replacing the entire engine and some other major things) everything is coming together. I'm actually pretty pleased with the way things are going right now... But there's one problem that starts to bother me more and more.

The braking is very weak. I have to kick the brakepedal down with a lot of force, usually even all the way to the floor, in order to brake. Even all the way on the floor i've got a feeling that the breaking is still not really strong.
I've had all the air remove from the braking system and was told by an expert that the brake disks are new, or at least good.

I was told that this problem is caused by a faulty brake booster, but someone else asured me that it has to be the brake hydraulic master cylinder.

What i eventually want is that my brake pedal comes 'up' somewhat (so i don't have to push it all the way down any more) and maybe that it won't take so much force to break any more.

Can somebody give me some good advise (on what part to replace) ??

Thanks a lot for any input !

- please forgive my sometimes crappy English, it's not my native language -

[bazar01]

If your pedal goes to the floor when braking hard it is not good. Better not drive the car until your brakes are sorted out.

Few things to check.
1. Wheel calipers/cylinders for signs of fluid leaking. Not sure if you have cylinders or calipers. If there is leak, seals are bad.
2. Brake master for signs of leaking. You have to remove the master from the brake booster and look for brake fluid seeping out from the back side. If there is, master seals are shot.
3. Check your brake hoses that go to the wheel area for fluid leaks too. Maybe your brake hoses are expanding when braking. Not sure but you can check.

To test the brake booster, with engine off, push the brake pedal until its hard to push and keep the pressure on the pedal then turn engine on. The pedal must come down about an inch when engine starts developing vacuum. If your master cylinder has leaked fluid in the back side for a long time, chances are it has damaged the booster diaphragm.

[strath44]

sounds almost certainly like a faulty master cylinder and or seals. I've had this prob a couple of times mainly on older cars.
I was advised that when bleeding brakes, if the person pumping the pedal uses to much (excesive) force that the master cylinder rod travels further than normal taking dirt onto or even reversing the seals causing faults.
Has work been done on the brakes recently?

[Shorty]

first off: this is not a recent problem. i've had this problem for as long as i have the car. Recently the brakes have been bled (if that means removing any air from the system).

The more i read about this, the more i get the feeling that it's probably the cylinder or maybe one of the cylinder seals.

It looks like something has been leaking where the booster and the cylinder meet :



The thing is that i've never had to add any fluid to the brakes (as far as i know, i don't do all the maintenance myself).

Thanks !

[Zero10]

I'd lean towards master cylinder, given that the seal between it and the booster has been leaking for a little while (from the looks of it).
If the booster fails, you will have an abnormally hard pedal, with very little travel, at least for the most part.

[rudidelange]

That looks like a bad leak. Overhaul the break master cylinder - if break fluid leaked into the booster it could affect the booster.
You can also remove the booster and check inside if the brake fluid entered the booster.

[Shorty]

Another follow-up:

It took me a while, but i checked the cylinder in the rear drums, it's not frozen.

When the engine is not running, the pedal is a good way up. When i start the engine, the pedal (pressure) drops a lot immediatly.

What does this mean ?

Is it clear now that i should replace the master cylinder ?

[Zero10]

It sounds like your brake booster is good. I'm presuming that you're talking about the pedal while you're pushing on it.

[Shorty]

yeah, that's what i meant.

So does that mean i need a new master cylinder ?


Does somebody know if PP carries a master brake cylinder for a '81 924? i can only find ones for 944's and unfortunatly they're not replying to the emails i send them.

[Zero10]

I'd be leaning towards the master cylinder. Even if it's not causing the weak braking, it's leaking pretty badly. Although, the general consensus on here seems to be that it is the master cylinder.
I'll try to dig up the part number, to check if it's the same on a 944 as a 924.


*edit*
nope, checked it out, yours has a stop light switch in it, as well as having some slightly different physical properties.
I was hoping they might match, would make your search a bit easier.

I looked, paragon products carries one that I believe should fit, but it's pretty pricey. Nearly $200.

[Shorty]

Thanks for looking anyway !

edit:
i can't seem to find the right MC on that site either.

http://www.*****************.com/porsche_924_944_968_brake_pad_rotor_part_s/103.htm

it looks like they've only got one for a 924S or a 924 Turbo. I've got a regular (european) 924

Did i overlook it maybe ? or do you perhaps know if either the S or the turbo one is the same as the regular 924 ?

Unfortunatly, i'm not to good over the phone, since English is only my second language, so just calling them about is not my first choice.

[Zero10]

Lol, I understand about calling them =)
I'm not 100% sure, but I think the 924 turbo one will fit. The PET lists them as different part numbers, but I'm not sure exactly what the differences are. Hopefully somebody else know a little better than I do.

[Shorty]

Originally posted by Zero10
Hopefully somebody else know a little better than I do.

Does someone know this ?

please ?