Porsche 959 Brakes sticking

[Scott_Tucker]

Hi guys,

I just started working on a Porsche 959 which has a brake dragging issue. When the brakes are applied and you come to a stop the brakes will stay partially engages for about 15 seconds and then will suddenly release.

I am taking over this project from another tech and have never worked on a 959 so I am unfamiliar with the system. We do have the manual and I am in the process of reading it.

Also, the front caliper pistons are different sizes. Does anyone know which ones should be the leading and which ones the trailing?

[RSTarga]

sounds like brake hoses

[GH85Carrera]

Wow, you are trusted to be working on a 959 and you are asking an on-line forum for help with brakes?

I hate to sound harsh but I don't know of anyone but Wayne that owns a 959 and works on themselves.

[KNS]

I don't see a problem seeking advice, there is an amazing reservoir of talent and knowledge here. I believe John Walker has posted once or twice looking for help...

[Bill Verburg]

Originally Posted by Scott_Tucker

Hi guys,

I just started working on a Porsche 959 which has a brake dragging issue. When the brakes are applied and you come to a stop the brakes will stay partially engages for about 15 seconds and then will suddenly release.

I am taking over this project from another tech and have never worked on a 959 so I am unfamiliar with the system. We do have the manual and I am in the process of reading it.

Also, the front caliper pistons are different sizes. Does anyone know which ones should be the leading and which ones the trailing?

smaller piston always leads

[Scott_Tucker]

There is no harm in asking. I'm not technically inept but thought there may be guys out there who have experienced similar problems and could lend some advice. I know there were only 250 of these cars built so the chances of someone knowing a 'common problem' are slim, but it is at least worth asking. If you never try, you are guaranteed to fail.

After talking to the guy that was working on it before I found that the following have been replaced:

Front calipers have been rebuilt.
Front brake hoses have been replaced.
No computer faults.
Valve blocks at front calipers have been disassembled and cleaned.
Valve blocks from another car did not fix.
ABS works intermittently.
No rust in system.

Talking to the owner, I found out that the reason the car was brought in was because the brakes failed completely. It was found that a check valve in the high pressure pump had failed. The problem surfaced after that. I would normally suspect that something went wrong there but have not gotten far enough in my diagnosis to determine that.

-Scott

[euro911sc]

Its not like 959's come into the shop every day! LOL

It may be as simple as the rubber dust covers getting old and firm, though.

-michael

[RSTarga]

The pressure switch on my c2 turbo failed and the brakes got rock hard, could be similar problem. Did the guy who rebuilt the calipers do it correctly? You are probably going to have to go through the whole system. The main problems occur because these cars tend to sit around. How about the master cylinders, are they returning?

[ChaseN]

Originally Posted by Scott_Tucker

Hi guys,

I just started working on a Porsche 959...

I'd help, but I can't seem to get past this line...I can only hope that I can (truthfully) slip that phrase into casual conversation one day...

[porsche930dude]

I know nothing about 959s but I would drive it around a while and maby itll fix itself. like others have said when a problem arizes its usually from sitting. The brakes on my truck and jeep do the same thing but then they are old rusty drum brakes .

[mnewport]

I had a similar problem once on an E38 BMW 740I. I know it's not the same kind of car, but the problem was caused by the brake master cylinder. Someone had accidentally put steering pump fluid in the brake system, and even though it had subsequently been flushed out, it still caused the master cylinder to seize up for 15-20 seconds when the brakes were applied.

[Flieger]

If all the brakes suddenly release at once I would suspect that the master cylinder is sticking. Possibly due to rust in the bore from water in old brake fluid that sat without using the car much.

[CHICKS]

My old bug did the exact same thing. It was the brake lines. Did you say you replaced the 20+ year old brake lines? Mine were so swollen, dental floss probably couldn't fit down the line....Do you have any pics to share with us?

[Kevin Stewart]

i bet its the abs is activated and not releasing, Kevin

[ratpiper71T]

I know nothing about 959s but I would drive it around a while and maby itll fix itself. like others have said when a problem arizes its usually from sitting. The brakes on my truck and jeep do the same thing but then they are old rusty drum brakes .

I was just about to mention the same thing happens on my 2000 Cherokee. I believe it to be the little piston cylinders sticking and or the brake lines swelling. It didn't happen when new. Sometimes it manifests itself like a clunk when taking off from a start.
Although not as fine a machine as the 959, my bet is on rebuilding the rear calipers and the brake lines. ABS could definitely complicate the issue...don't know if 959s had ABS. Is there a proportioning valve that controls front to rear pressure???

[Geronimo '74]

I agree with Glen.


And I hope for the guy that actually owns and drives that thing, THAT YOU GET IT RIGHT....

Would be a akward to find out that somebody who is 'not mechanically inept' had been working the brakes and messed up, especially at 170mph.....

I recommend professional assistance....

[andyt11]

Where are you going to find "professional assistance" with experience on a 959?!

Its just a brake system, it cant be that unique. He'll just have to do some reading and work through the steps logically.

Don't overlook things that have already been replaced / rebuilt. You can waste a lot of time assuming a particular component is good. Good luck with it.

[Geronimo '74]

Originally Posted by andyt11

Where are you going to find "professional assistance" with experience on a 959?!

how about an official dealer??? The brakes are the same or similar to the ones on a 911 turbo plus the anti locking system.

Either way, if I had a 959 that needed brake work, I'd be looking for more than

am taking over this project from another tech and have never worked on a 959 so I am unfamiliar with the system. We do have the manual and I am in the process of reading it.

But hey, it's only brakes, not that important at all...
I wouldn't let an inexperienced guy work on the brakes of my Datsun (if I owned one) but that's just me.

No to be disrespecting anyone, really not.

[carr914]

Maybe call a Tech @ Canepa - They work on 959's all the time

[RSTarga]

+1 on Canepa