Went to get the 92 ready for spring. In doing inspection of brakes the rear brakes are not even engaging. Pulled the brake line, pumped the brakes, only drops of fluid coming out.
In pumping the brakes I also found that the brake pedal is rock hard when car is not running. It is still hard when running, but it does move more. Another possible symptom is that the anti-locks seem to kick in under normal driving. I suspect this could be due to the rear brakes not working at all.

I am trying to figure out what is exactly wrong. I have done some tests to see if the brake booster is working properly and it passes all three with the exception of the pedal being so difficult to push.

Test 1
1.With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
2.Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.

Test 2
1.Run the engine a couple of minutes.
2.Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Test 3
1.Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.

Lots of posts on brake booster issues with C4s, but with the no fluid problem at the rear I am wondering if I have more issues with the master cylinder as well. Could this really just be a vacuum issue with the hoses being 22 years old?

I have yet to disassemble the master cylinder or booster as I would like to only do this once, and only replace what is needed. If the booster is bad, could that account for no fluid going to the rear brakes?

Found this great post on how to replace the booster. Along with other posts sounds like a half day job.

Thanks in advance for any input.

 

If I had to take a guess it almost sounds like the proportional valve is not centered which would account for no brake action in the rear. The 92s historically don’t have a history of clogged ABS units as do the earlier C4s.

Take off the sensor on the side and see if you can see the valve or the step of the valve. It should be deep as the sensor goes in full for normal. Is there a brake light on the dash? If it is out of position, you may be able to move it with a small screw driver.

My feeling or opinion is you are going to find an issue with the master.
Does not sound like a booster problem and since both wheels are affected, don't think it's the ABS pump unit.

 

I do not see any cracks in the booster like many have reported on the forum unless it is on the underside where I cannot see. No brake light on the dash.

I will see if I can reach and view the sensor area. I suspect this may be easier to view/adjust by removing the master cylinder.

 

If you unbolt the master, it can come forward several inches without taking off the hard lines. The sensor is passenger side of the master with a wire on it. I would give it a shot before removing the hard lines.

 

Tried pulling that, could not tell if it was right or not. Ended up pulling the hard line for the rear brakes. Not a drop of fluid comes out. I suspect that this helps prove your theory that there is something wrong with that valve inside the master cylinder.

 

Couple other questions for any brake experts out there.

Is it possible that if the master cylinder is defective it would cause the hard brake pedal? (i.e. not allow the components to move thus preventing any fluid from flowing to the rear outlet)

If I remove the master cylinder from the brake booster, will pressing the pedal tell me anything about its functional status?

 

I ended up replacing the master cylinder after doing a test bench bleed and not seeing any fluid flow from the rear outlet.
I also found that the brake booster was fine. The vaccum hose was not a very tight fit at the intake. I cut about 1" off the end and replaced it and the pedal was 100% better.

 

Are you fixed or not?
Did the new master fix the problem or not?
Is there pressure and fluid at the rear calipers?