Is master cylinder shot?

[burban1500]

I jumped in my 89 burban today rolled down the street and lightly hit the brakes to come to a stop when all of a sudden their is an abrupt bang causing the rear brakes to lock up. Now every time i touch the brakes it does this, it doesnt matter at what speed. The brakes have been fine for about 2 years when i did my 1 ton swap, new calipers on the 60, new brake lines all around and new shoes on the 14, and many brake bleedings. The truck stopped on a dime. truck is a half ton with original half ton MC and booster. Is the Master finally shot?

 

[Fordum]

Don't think it is the master. Sounds more like something bad wrong with the booster. A bad MC will give you soft brakes or brakes that will not hold. They don't get stronger<G>.
Could be the proportioning valve letting all the fluid go to the rear. Does it lock up both rears?
If it only locks up one wheel, you need to check that wheel out quick. Both wheels are more likely to be something up front. Actually, I suppose if you were running a spool or a really aggressive locker, a brake problem on one rear could lock up both....

J.

 

[burban1500]

yea its locking up both, gently touch touch the brake pedal and then their is a slam which leads to both of them locking up

 

[GNERGY]

I say there is something broken in one of the rear drums, making them lock up.
Tarey

 

[Fordum]

I keep thinking that, but unless he just redid both rears, I don't think he would have an identical failure on both.
Oil on the shoes, would do it, but it sounds too sensitive to be oil. He says he just barely touches the pedal and bam! they lockup.
Sure sounds like the control valve in the booster has failed and is sending full boost as soon as he touches it.
Of course, that does not explain why the fronts are not locking up.

 

[GNERGY]

Says he redid them 2 yrs ago?
Tarey

 

[Fordum]

Try pulling the vacuum line off the booster, and plugging it. Then hit the brakes a couple of times in case the booster is holding vacuum.
Then, crank it, aim it at something you don't mind hitting, dirt bank, long uphill slope, etc. and see what happens when you try to stop it. I suspect the brakes will behave themselves. Of course, they will be harder than diamond without the booster, but they should not lock up.
If they lock up then, its not the booster.
Note, without the booster, you may not be able to stop it. So, know what is in front of you before you start.