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Re: Spongy Brakes
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Posted by Andy [Email] (#759) [Profile/Gallery] (more from Andy) on Mon, 02 Oct 2000 19:11:57 Share Post by Email
In Reply to: Spongy Brakes, Scott Paterson [Profile/Gallery] , Fri, 29 Sep 2000 21:00:06
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I had a bent rear backing plate on my '67 96 V-4 from the time I bought it used. I always had trouble getting that wheel cylinder to bleed completely evne when a new one was fitted and I finally realized that it was because the pistons were cocked in the cylinder and air was being sucked past them when the pedal pressure was released.

The spongy, sinking pedal is usually indicative of internal leaking past the piston in the master cylinder. Look for leakage down the power booster below the master as leaked fluid may be coming out the vent at the back of the master. The rear wheel cylinder is certainly a place to start if there is obvious leakage, but remember, if you fix one leaky cylinder, the next weakest is likely to begin leaking before long. I would at least rebuild both rear cylinders and probably the master, too. The front cylinders aren't as likely to leak; they are more likely to get rust on the piston that causes the piston to stick when the pitted spot tries to pass over the seal. New pistons are probably next to impossible to find. I'd check with Chip Lamb as he seems to have sources. The SAAB manual says to clean the pistons with brake fluid before sliding them back into the caliper when changing the pads. I always did this and never had any trouble with my calipers. Use an old toothbrush (not a steel wire brush) and gently scrub the dirt off. Get the piston as clean as you can.

A shop I worked for in New Hampshire years ago had new pistons for a Rover sedan made at a local machine shop, but the guy made them from aluminum and the first time the brakes were applied they expanded and locked up tight inside the caliper bores. Bad idea. They eventually had the original pistons re-chromed. Maybe there is someone with some stainless stock; you'd never have rusted pistons again.

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