Yesterday my '78 ate its third master cylinder in 18 months. This one was less than two months old. Not a good feeling to be going down the highway press the brakes and have nothing happen but the red warning light illuminate (I didn't need a warning light to tell me I had a brake problem!). I was able to slow and stop the car without using a building for a backstop.

Other than a a coincidence of having several bad master cylinders installed on the car, would there be any adjustments to the piston rod from the booster that could affect the master cylinder negatively? I'm kind of at a loss to explain the run of defective master cylinders.

Hey Bruce I have to ask this, is it still full of brake fluid when the brakes go out?? I'm also having a brake problem. My fluid is going away but I can't find a leak anywhere.

It turns out the problem wasn't the master cylinder, though it had all the indications of it...no pedal, no apparent fluid loss. It ended up the right front caliper seal had caused the problem. Instead of leaking fluid, it was sucking in air. It allowed a lot of air in suddenly and I lost nearly all the pedal.

Fixed now and stopping fine.

All I can say about your car losing fluid is there has to be a leak somewhere as it's going someplace. Just because you don't see fluid doesn't mean it's not there. A small leak is enough...it doesn't have to spray all over everything. The best I can say is thoroughly check each caliper around the seals, the hoses and metal lines. If everything there is OK, it may be the master cylinder leaking into the booster.   You may have to unbolt the master cylinder and look at the rear of it and see of that seal is leaking.

Bruce, I know the feelin... I went thru a similar problem on my 'Burban a few years back. Put five/six masters, some re-built, one NEW($$$), some cast-iron, some aluminuminum. Still couldn't have a good pedal more than a few days, then to the floor.
In that case, it turned out to be my habit of mashing the e-brake every time I parked. One of the pushrods for the rear brakes was sticking to the shoes AND the piston, and every time I pushed the e-brake, it would pull on the wheel cyl. piston, suckin air into the system. Only took me a year to figure that one out!

That's not gonna be your problem, tho...just tellin a story...

Some brake pushrods are adjustable, but I don't know if a 78 would still be. I remember some older set-ups that were, tho. I'm assuming you're NOT losing any fluid, just pedal?
It is possible to get several bad remans in a row, if they are coming from the same supplier. If the cylinder is not nice and pit-free when they rebuild them, it will fail sooner or later. If the pushrod is a wee bit too long, it could damage the seals inside the master, but normally, that scenario also causes the brakes to drag a bit, kinda like riding the pedal all the time, creating drag on the brakes. It doesn't sound like you're having that problem, either.

If you are using reman cylinders, I would have to suspect that as the root of the problem. If you are gonna take the one you have now off, why not take it apart, and see if the inside of the cyl. shows any previous corrosion/pitting? That would tear the seals up pretty quick, depending on how much drive time is on it.

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Joel Adams
C3VR Lifetime Member #56    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

ok...disreguard my last post... Glad you got it figured out, Bruce!

I was gonna mention the same thing on Ken's problem...just because you don't see an obvious leak, doesn't mean it ain't leakin! Take the master loose from the booster, and see if the rear is wet with fluid. If you don't have power brakes, you can look under the dash, up where the pushrod comes thru, and see if it is wet there. If the master is leaking(non-power), it will run down on the inside of the car...the one place you DON'T look!

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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

I'm glad it was figured out, though it would have been nicer on the wallet to have had the master cylinder replaced rather than the caliper. At least the car has a good, hard pedal now.

Pressing on the brakes with almost no response is not a good feeling. This is the second time it's happened to me...20+ years ago in a different car and that was a master cylinder failure...single master cylinder as well in a 1963 car. I'm very fortunate both times I was able to bring the cars down with no ill effects. This time there was enough pedal to slowly bring the car to a stop...the first time it happened at low speed and I drifted into a parking spot and the curb stopped the forward motion. Hope there's no third time.

Some issues can wait for a convenient time to be corrected...with brake issues I don't mess around...they get tended to immediately.