Yesterday, on the way home from work my brake pedal went to the floor. I had some braking... but not much. Good thing I was not motoring along at a good clip on the highway. Well, I made it home alright and found out along the way that if I pumped my brakes several times I would get some pressure, but it would not last.

I have had quite a few things to take care of today and I will get under the hood tonight. I have been to the various sites to see what the problem might be (including 928 Tips, the Rennlist's brake threads) and I think that the master brake cylinder may be shot. The car has 130k on it and the warning lights never came on. I am also one of the induviduals waiting for the next printing of the set of manuals (mine has been on back-order for about 3 months), so any help or tips will be appreciated. Does this sound right ?

Thanks <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

Rob

 

Pull back your carpet under the brake pedal and see if there be some fluid soaking everything. What is the level of fluid in the resevoir?
Have you crawled underneath and peaked at each caliper?
5-speed or Auto? Clutch system uses brake fluid from same resevoir.
Inspect everything first. then place your parts order. Keep us posted <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

 

Joe,

Negative on all counts. No apparent leaks coming from anywhere, the calipers all seem normal & dry, no puddle or wet spots in the car (my car is an auto, by the way) and nothing out of the ordinary on or near the brake master cylinder.

The only item that had any look of being in need of a replacement was the black cylinder type thing in front of the brake master cylinder with the 2 wires coming out of it. The image shown is from Tony's V1UHOH website, under the "What is it ?" section and even he does not show what the item is. It is number 10 in the image. On mine, the terminals where the wires plug into (the light tan items) are cracked and look rotten.

My question, is it possible that something can go bad inside the brake master cylinder and still look fine on the outside ? The reservoir still has the correct level of fluids in it also.

Thanks for any help or thoughts on this matter.

Rob

 

Joe,

Negative on all counts. No apparent leaks coming from anywhere, the calipers all seem normal & dry, no puddle or wet spots in the car (my car is an auto, by the way) and nothing out of the ordinary on or near the brake master cylinder.

The only item that had any look of being in need of a replacement was the black cylinder type thing in front of the brake master cylinder with the 2 wires coming out of it. The image shown is from Tony's V1UHOH website, under the "What is it ?" section and even he does not show what the item is. It is number 10 in the image. On mine, the terminals where the wires plug into (the light tan items) are cracked and look rotten.

My question, is it possible that something can go bad inside the brake master cylinder and still look fine on the outside ? The reservoir still has the correct level of fluids in it also.

Thanks for any help or thoughts on this matter.

Rob

 

I would replace the brake fluid. You say that you can pump up some pressure, which doesn't last. This could be air in the system. It can also be a leak, but you haven't found any sign of it. Changing the fluid is periodic maintenance in any event- some fluid is hygroscopic and will get spongy with use.

 

The hydraulic system is a closed system. If you press on the pedal, and there is is fluid in the system, with no air, there must be resistance, UNLESS:
- The pressurized fluid can escape from the system somewhere - and that will usually be visible.
- The pressurized fluid can move a caliper piston enough to absorb the pressure completely - and that would require a LOT of pad knock-back or something similar. In addition, after the excessive movement is taken care of, the pedal will resume its normal firm feel.
- The master cylinder piston cups are worn or damaged enough to allow them to move freely thru the fluid rather than pressurizing it.


My guess is a bad master cylinder. It might be worth bleeding the brake system, then CAREFULLY trying it out - but I wouldn't trust it at all to start with.


The cylinder in the photo (#10) is the connector for the ABS sensor on the left front wheel. No connection to the master cylinder. What looks like tubing is electrical wires.

 

Donald & Wally,

Thanks for the comments & suggestions. This actually gives me the chance to get some of those nifty "speed bleeders" and "power bleeder" that you sell Wally. I figure that every time something goes wrong on the car, I can then purchase some new "special-built" items to stock my garage. Do you also sell new master cylinders ?

One other thing, I have a set of manuals on back order (I think, along with most other 928 owners...). Would someone be able to copy just that section of the maunals for me to see just how to do this. This will be my first "big" repair job on my 928.

Thanks to everyone !

Rob <img src="graemlins/wave.gif" border="0" alt="[byebye]" />

 

Hey Robert, don't over look the brake "booster" #5 in the picture. Internal failure can cause the same symptoms. Some one please correct me if I'm wrong.

Anthony Tate
79/928 Silver Metallic <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />