Third Master Cylinder no Good?


Greetings all! It’s been a little while. Hope no one’s missed me too much! =p

Here’s the deal. I was driving home today and I started to notice a little free play in my brake pedal. There’s normally a very tiny amount of course, but this was larger. Normally there’s about 1/8th inch, then it became closer to a half. It was negligible so I ignored it. But as I went on, it got a little bigger, until almost half the pedal swing became a dead spot. Bothered, I drove home.

What I noticed on my way home that I hadn’t noticed before was that there was significant drag on the car. It barely moved at idle in drive, and took more power than normal to accelerate and cruise.

I checked all four wheels seperately when I got home. The drag was the same on all equally, so that ruled out a single seized caliper or colapsed line. So that lead me to the master cylinder. I figured there wasn’t much that I could do to check it.

So I went to move the car back to its normal parking spot and the problem was gone. The pedal was high and normal. No freeplay. No drag. I drove a three mile round trip and gave the brakes everything from a graze to a slam. Everything was fine.

This is this car’s second replacement master. Third overall. This particular master is brand new, installed two months ago and has seen only a few thousand miles.

One last thing to add. I remembered experiencing a simmilar problem a little while ago. The freeplay/travel started to crop up in small amounts, but never got much more than negligible. No drag. And it was gone the next time I got in the car.

So my theory is that the master cylinder pistons are binding up some, but only after some time. Today I was out on the road for over three hours, and driving in traffic on stoplight highways. Constantly getting up to 40 and then having to stop. I’m normally a highway driver so I only have to use my brakes occasionally. Maybe it’s just the car’s way of reacting to the race weeks traffic. =)

Any ideas?



Just a suggestion to perhaps narrow it down…next time the brakes drag, try loosening the nuts/bolts holding the m/c to the brake booster unit. See if the brakes free up. If so, likely there’s a mechanical cause that’s not in the m/c. You may find (like a '78 Monte Carlo that I remember too well) an adjustable rod in there, adjusted a trifle too long and keeping just a little pressure on the m/c piston. HTH.


Just a swag, booster?


Sounds like your brake fluid uses the flubber additive. I’ve been lucky with brakes and because of that, I have only had to change one master cylinder of my own and it worked! Lucky doesn’t give much experience, but the experienced will get lucky after awhile. Keep at it.


The car’s a 1990 Riviera if you hadn’t guessed.

The booster is not adjustable. It’s also fairly new. It’s a rebuilt unit that was installed about 5000 miles ago.

I removed the master to check. The pushrod seems to be sitting where it should be. If and when it happens again, I’m going to try and pull the master before the problem disappears. I figure if the master is the problem then the primary piston should be slightly depressed. A reasonable assumption? (This is a step bore master.)