Katharine

My 93 964C4 brakes have a problem! They work OK, however, the brake pedal does not return to full position immediately. Rather, it returns normally except for the final 1.5 inches. Then, it s-l-o-w-l-y returns to full position. Evidently, these are complicated brakes! The mechanic said he had heard of this problem only once before... on an RS America... which is not a C4, so??? He thinks it may be one of the 3(?) pumps on the system... over $1200 for the part. Ouch! Any info anyone? I just got back from Summit Point track and leave for Pocono on Friday. I CAN drive it this way, but would prefer not to!

garrett376

Did this happen only after you drove at a track event? If so, you may be looking at a simple bleed. If a pump was having a failure, you'd be notified by the PDAS and Brake and (!) warning lights. You're right - the C4 and RSA have different brake setups - different assist mechanisms, and totally different master cylinders.
I'd be betting you need a bleed, especially after a track event!

garrett376

Additionally, there is only one hydraulic pressure pump, and one master cylinder, and 4 brake calipers - those represent all your "pumps" - I am not sure what your mechanic was referring to, but give us some more info about this (warning lights, other findings) so we can help ya' out!

Katharine

Not from a driving event. First noticed it in February/March. The mechanic has bled the entire system, taken the brake pedal area apart, to determine if any mechinical cause, such as friction, linkage etc. One thought, but probably not valid.... This is a cab... when the cartop guys unhooked the battery, they dinged the pump that sits against the front bumper area... brake fluid all over! So my guys had to install a new pump. But that was last Fall... so how can that be related?

Adrian

This problem is purely mechanical. The brake pedal is connected to the master cylinder pistons by a push rod. When you release the pedal the master cylinder and the return springs should return the pedal to the correct position by forcing the push rod back pushing the pedal up.
Air in the normal brake circuits (front or rear) will cause this problem. I recommend a full and proper brake circuit bleed be carried out again.
The master cylinder is part of the normal brake circuit. There is no pressure fluid from the hydraulic pump provided to the brake circuit.
You might also want to check the brake circuit fluid levels. The brake fluid reservoir is divided up into chambers.
The master cylinders of the RSA and C4 work in an identical manner. Their difference is in piston diameter and the assistance force provided.
Ciao,
Adrian.

Tom W

I know from experience that you can have a "pump" problem and no indication by any warning lights. I don't think it's the case here as the symptoms don't match. My ABS pump was bad and the only symptom was a variable and "soft" pedal. A new ABS pump is about $2600 but my mechanic found a salvage unit for $300.

Adrian

There is no pressure warning system for the ABS pump in either the C2 or C4. This is the brake fluid return pump and the only available warning is a general ABS warning light if the pump current draw is too high.
The ABS pump (return pump) only activates on the 964 C2 and C4 when the ABS activates.
How the ABS pump can cause a soft pedal escapes me. I helped build a test bench for these puppies and we could duplicate a lot of failures but that motor even when it was kept running all the time actually had no impact on the system. It spends most of its life not running.
An open solenoid dumping brake pressure would cause a soft pedal but the return pump just sends the fluid back to the master cylinder when the ABS is activated.
The hydraulic pump for the brake boost requires two failures not to provide a warning. The pump and the pressure switch and when this puppy stops working the brake pedal is too hard to depress.
Ciao,
Adrian.

Red rooster

Good suggestion to pull up the plywood cover , then disconnect the pushrod and check the pedal for free movement.I have seen the pedal bushes go bad and give a bad pedal return.The bushes cost cents!!and are easy to replace.

Katharine

I'm confused. There is a pump just inside the front bumper. And, the indicator lights include one for "4-wheel"; for "park brake; and for "brake", as well as others. The book states that this light will stay on until the system is fully pressurized - is it a nitrogen bladder of some sort?!?!? I assume, then, that this pump pressurizes the ABS system. So, I drove Pocono International Raceway (1/4 the bowl plus infield) this last w/e with the pedal still not returning full-way immediately. It wasn't that much of a problem... and should stop my "double-braking" habit! Any confirmatory - or other - thoughts would be appreciated.

garrett376

There's an accumulator that has a pre-pressurized state, but that's what gets pressurized and hold pressure like a reservoir.

It pressurizes the hydraulic assist, and the PDAS system - not the ABS, which is what I think Adrian was getting at.

Adrian

What book are you reading?

The pump inside the front bumper is for the brake boost system and the traction control differential locks. It has NOTHING to so with the ABS which is part of the actual brake circuit.
The pump pressurises a hydraulic accumulator which provides the pressure fluid to the brake boost and traction control system.

The brake warning light has two functions.
1/. To advise you that the brake boost is not functioning because if this fails you will not be able to depress the brake pedal and
2/. To advise you that the brake fluid in the reservoir is low.

One warning comes with the warning horn and the other does not.

There is NO direct link between the high pressure pump and the ABS. The brake system itself is pressurised by your foot pressure acting upon the push rod which operates the master cylinder pistons mechanically assisted by the brake boost circuit.

Your problem is directly related to the brake pedal operation. Try bleeding the brake circuits as has been recommended.

There are NO thoughts on how the system works only how it works.
So just to repeat. There is NO direct link between the pump forward of the battery and the ABS and the brake circuit.
Ciao,
Adrian.

sph

Hi Adrian

I found my 92 964 turbo look cab has the syptom of number one you mentioned, the signal (!) on recently when ignition. Sometimes, before running the car, it's difficult to depress the brake feeling like no power assist. But the brake work again after a while. Please advise what exactly the pump called that I can get one from dealer. Thanks.

Don

bhensarl

Katharine and sph,
If you guys don't yet have your hands on a copy of Adrian's book, you definitely should. You can see the name in his signature,
"Porsche 911 Enthusiasts Companion 'Aufstragsnummer 964'"
Do a search on Amazon.com or a similar site and pick one up. It is the resource for information on your 964, and comes very highly recommended on this board. I know I've gotten my money's worth many times over.
Brian

sph

Hi Indycam
My one is a C2, I am located Hong Kong. The situation worries me cos the (!) signal on and off from time to time. I was told the problem arises from the ABS pump after the engineer checked my car's computer system, saying the pump is getting less effective. Of course, I still doubt so checking with other enthusiatists.

Cheers

Don