I recently ran across a helluva problem regarding brake performance on my '88 Turbo 2, non-AAS, non-ABS car. Here's my story:

I bought this car to build from the ground up (and I still have a lot of work to do). Aside from the interior completely missing, a water jacket failure on the front iron, and a small colony of ants living somewhere in the frame, there was this brake problem...

Basically, it went like this: There was a very good, solid brake pedal as long as the engine wasn't running. As soon as you started the engine, which applied vacuum to the booster, the pedal would easily sink down 3/4 of the way, at which point it would finally engage the brakes. Sounds like a very simple case of:

A) Master Cylinder internally bypassing under high pressure.
B) A weak brake hose expanding under pressure.
C) Spongy, contaminated brake pads or faulty hardware.

Well, things could never be so simple, or so I thought.

I replaced the master cylinder 3 times, and finally ended up rebuilding my original myself. I was completely convinced it was a bypassing piston. I put it in a press to operate it, and attached my brake pressure gauge to each port, blocking off the other two. At no time did it ever bypass, even up to 5,000psi. The master cylinder was good. Re-installed, problem persisted.

Brake hoses. Surely one of those old rubber lines broke the nylon, allowing it to expand under pressure, right? I replaced them with Mazdatrix braided stainless lines. Same problem.

Pads. Maybe one of the pads is swollen or separated, causing the sponginess. Not a chance; All 8 were replaced, exact same problem. All hardware replaced, too. Same problem.

I'm starting to become confused. I don't understand. I swap the front calipers for a known good pair. Same problem. Swap the rear calipers and the same problem. I replace all 4 rotors, same problem.

I'm at my wit's end; I've tried everything by-the-book. I was desperate. As a last resort, I replaced the brake booster. Problem solved. WHAT?!?!? I don't understand it myself, but it fixed it. Completely. I'm going to take apart the old booster to find out why it failed and how it can create a spongy brake pedal, but only when it has vacuum applied, and without dragging any brakes.

Good Luck!

Black_Sunshine

 

Sounds like Air Somewhere. bleed the lines on all four corners? Did you replace the Booster itsself?

 

'm at my wit's end; I've tried everything by-the-book. I was desperate. As a last resort, I replaced the brake booster. Problem solved. WHAT?!?!? I don't understand it myself, but it fixed it. Completely. I'm going to take apart the old booster to find out why it failed and how it can create a spongy brake pedal, but only when it has vacuum applied, and without dragging any brakes.


eh hem

 

^^ (Not cmanns) did you read the whole thing through?

 

Ants???

 

either the mechanical linkage inside the booster broke,
or the push rod came out of adjustment,
or, the push rod slipped off of it's seat inside the booster.

-Pat

 

did you at any Time,take the rubber hose off the Booster?(at the Booster and the steel line that runs to the Passenger side and gets vacuum?).
That little "ESS" shaped Hose has a Check valve in it.You can get a nice hard Pedal,But when you Put the Brakes on,it is like you have no Brake Pressure!.You literally have to grab the steering wheel and FORCE the Pedal down for the brakes to work//Does that sound Familiar?.if not..Just Posting,for the heck of it.
It has to be put on One Specific way,so that it Gets vacuum when the Brakes are applied..
If that is the case,then just Switch the hose over(turn it around 180 degrees).