Problem: Brakes locking up

[slick9997]

I had to put a new master cylinder on. Two days after I was driving home. The Pedal got where I could not mash and the brakes locked up on all four wheels. When Igot home had smoke coming out around all four wheels. I jacked it up and
could not turn tires with hands. Had to open bleeder valve to release the brakes.
Has anyone ever had this problem?

[rage'nrat638]

never had it happen to me ..but it sounds like the master cyl is not releasing..
you got a bad master cyl.. take it back.. hope you saved the receipt

[e rock]

You need to adjust the pushrod. Like was said above, the master cylinder isn't releasing fully.

You should be able to get away with just shortening the pushrod at the pedal. I'm sure there's a factory spec for how much freeplay it's supposed to have, but you just need to make sure that the pushrod isn't exerting any pressure on the booster when it's released.

If that doesn't help, there's a slight chance you may have to adjust the pushrod from the booster into the master, but that's relatively rare and a lot more complicated

Good luck,
Eric

[Longhorn Man]

e rock, that's actually how it's supposed to be done on a power brake truck... under the master cylinder.

[slick9997]

I do not have power brakes This is the second master cylinder to have this problem in a week. Is it possible to ge two bad master cylinders in a row?

[kbs71]

Try what e rock said. unbolt the MC from the firewall (DO NOT disconnect the brake lines). Now try to hold the MC up in place. does it fit flush against the fire wall or do you have to push it down or did you have to get it to seat by tighteneing down the bolts to the firewall. It should just fit flush againt the fire wall with out any pressure being exerted to get it flush. If it doesnt you need to adjust the brake pushrod. Under the dash you will see a rod and clevis end attached to the top of the brake arm. you will have to break the lock nut loose and adjust the rod shorter till you have @ 1/2" to 3/4" play in the pedal before the rod end touches the cup in the MC. You don't need to remove the rod, just turn it by hand with the clevis still attached to the brake arm.
My experiance has been to loosen the rod end (clevis) quite a bit, till i had about an 1" or more slop in the pedal when the MC was snug up to the firewall with the nuts. Then turn the rod till I get about 3/4" to 1/2" of play in the pedal before the rod touches the cup inside the MC. You can "feel: this by keeping your hand on the rod while you move the pedal and you will feel the rod stop. Then tighten up the lock nut, the MC nuts and try it out.

[slick9997]

I loose the nut on the rod and it was not putting any pressure on the MC left it lose. I can mash the brake pedal 1/2" before it touches the piston in MC will drive it to see if helped. Thanks for the help

[kbs71]

If it wasn't putting any pressure on the MC then that probably wasn't the problem. I can't remember but do you have a combo valve attached right next to the MC. It could be that also. If you still have the problem and you have that valve try bypassin it. this would mean buying fittings and bleeding the brakes but it would rule that out. If you dont and still have problems, my very last thing I would try is to bleed all the old brake fluid out of each wheel cylinder till fresh fluid comes out. Otherwise it's another MC change.

[e rock]

Originally Posted by Longhorn Man

e rock, that's actually how it's supposed to be done on a power brake truck... under the master cylinder.

Ahh. Thanks. Sometimes things I 'know' are not true on all vehicles. That seems especially true on this truck...

Originally Posted by slick9997

I do not have power brakes This is the second master cylinder to have this problem in a week. Is it possible to ge two bad master cylinders in a row?

That makes me think you have another problem. Have you changed anything in your brake system? Converted to disk brakes maybe? Anybody know where the residual valve is located (i.e. is it in the m/c or inline somewhere else)?

Just thinking out loud...

[watahyahknow]

could be he has a master cylinder for diskbrake(s) yes

[71chevytruckguy]

Originally Posted by slick9997

I do not have power brakes This is the second master cylinder to have this problem in a week. Is it possible to ge two bad master cylinders in a row?

yes it is. when i replaced the on my '71 2 years ago the first one i got leaked from the rear piston seal, returned got anothe one and did the same thing. returned that one and 3rd time was the charm. when i had to do on my '69 rectnely, the first wouldn't bench bleed and the second one was the leakin fron the rear also. 3rd time was the chram, again. the reman master cylinders are hit or miss. chris.

[kbs71]

Hey Slick, did you get it fixed?

[cgoodwin]

Had the same problem when I switched to power brakes. The booster was fine and the master cylinder was fine. Both new items and came as a unit already put together. The problem was it was the wrong master cylinder. On my 71, what was happening was the system built up pressure while I was driving. The center of the master cylinder was the wrong one for the truck. It had to be a dappered center. Got the correct master cylinder and no problems since that time. Hope this helps from my experience.

[PanelDeland]

Not all parts are created equal.I replaced a wheel cylinder once and could not get one of the bolts started.Spent about 20 minutes and finally pulled it back out and found 'No threads in the mounting holes"

[slick9997]

Have not got it fixed yet. I went to the parts store and got them to order me a new one. I don't want try another rebuilt one. I will let you know if that fixes it. Thanks for all your help

[rage'nrat638]

sounds like your doing the right thing..let us know..your pal mark