The Stovebolt Site Forums The Parking Lot Big Bolts brakes locking up

I've got a '68 C-50 dump I use for work and the brakes keep building pressure and locking up. I release the master cyclinder and they work again, for a while. This doesn't happen all the time. Short of taking everything apart is there something I can do? I've replaced the booster and master cylinder.

Check the rod from pedal to master cyl if its too tight will cause drag. They need a little "slack", welcome to the Stovebolt, Brian

Thanks. I'll try it tomorrow. I've seen someplace where they say to change to fluid. Think that has anything to do with it?

Hi you did bleed the brakes when you changed the master and booster, so you already did a partial fluid change which I don't think has anything to do with your problem. Do you still have your old components if so compare push rod lengths piston counterbores to make sure they are the same. If you don't have them remove the master from the booster now reinstall the master, no fluid should be forced out of the master when reinstalling if fluid is forced out there is insufficient clearance between the booster and master and your brakes are partially on and can never fully release, good luck

I suspect it is the rubber brake line from the master cyl. (it is on the frame right behind the left front tire) over time the rubber swells inside the hose.
when this happens it acts like a 1 way valve.

Why would it only happen some of the time?

when you apply the brake there is pressure, the fluid is not under pressure when it returns. Could be just a piece of rubber is getting caught once in awhile.
Just pull the hose off and try blowing thru it both ways. You have to start somewhere and this is the cheapest spot.

tony b,

You said you changed the master cylinder and booster. Was it locking up before, and that's why you changed these items? Is your booster located under the truck, seperate from the master cylinder?

Stuart

The booster is attached to the master cylinder. I replaced the booster because it sounded like a blew a seal one day and then I had no brakes. I left the original master cylinder on orginally but my brakes were quickly locking up. Then I swapped out the master cylinders and they worked fine for a while. Now every onece and a while they keep building pressure (with every use) until the pedal is hard and I can't go anywhere.

I haven't tried any of your suggestions yet (but sure appreciate them) as it is 30 degrees and snowy right now.

Sounds like there is something acting like a check valve not letting the fluid back after you step on the brakes.

Mike

I have the same problem on my truck (48 2-ton) I just bleed off the brakes every couple of weeks and keep the master cylinder topped up. Let me know what you find.
Reuben

I just thought of another possible cause for the brakes locking up...the rubber flex lines! They have been known to swell shut on the inside while they can look great on the outside.

Just a thought!

Mike

I shoulda thought of that

OOPS!...lol !

I should have read ALL of the posts before I made my last one...duh !!! :rolleyes:

Sorry Don...

Happy Holidays!

Mike B

Still buried under snow right now but I sure appreciate the tips.

either the adjustment rod is run out to far or the the piston is binding in the master cyl.i would check the adjuster rod first anothe rpossibility is the rubber hose on one or both front wheels is swelled shut on the inside and acting as a one way valve

My 48 had a bad hydrovac causing the brakes to stay applied for a short while. Did you replace your booster with a new or used one?
Scott

New one. New master cylinder too. The weather still isn't cooperating so I haven't done anything to it yet but ran a couple of gravel loads the other day and it worked fine. Do you think the temperature has anything to do with it?

If it started after the booster replacement, that's where I'd look. Is the rod on the front of the booster adjustable and did the instruction sheet that came with it say anything about checking the length against the old one?
You also said
What method of release are you referring to???

If the truck is original it proably has a hydrovac unit under the cab floor on the drivers side attached to the frame,you will want to look for this part and if it is there rebuild it using the guide in Tech Tips.If this is the unit you replaced then the problem could be the valve located just below the master cylinder on the firewall that the vacuum hose from intake manifold is connected to.This is a one way valve and has all kinds of effects on the hyrovac brake system.I remove mine about once a year and blow through it with 40psi compressed air as a preventive maintenace sorta thing.That valve will pop like a shotgun if it is blocked and it will flutter if it is partially blocked.There is no info on this valve in the service manual which indicates that it was probably a non serviceable item.
Hobert