Rear brakes locking up....

[wyattglock]

I have totally rebuilt the brake system in my truck, except for the one freaking rubber hose on the rear. I am thinking that is going bad, but before I tear into them I wanted another opinion. When I first start the truck after sitting for a couple of hours, the rear brakes lock so hard they throw me forward in my seat. After 5 or 6 cycles of the pedal, the problem goes away and is fine until I let it sit again. I have bled them several times, and do not get any air out of them. Other than that hose, I can't come up with a reason why they are doing it. I am not losing gear fluid or brake fluid, so I can't say it is that causing the normal pressure problems. Any opinions are appreciated.

[Desert1957]

I know this will sound stupid but check the surface of the drums. My truck when cold would do the same thing , but after having the mirror finsh (glazing) machined off the drums it was fine.

Desert

[kelsfine69]

Greasy finger = Grabby brakes...try a can of brake clean on drums and shoes first...Might be master cylinder?

[rwhit57]

There is a proportioning valve down by the core support. I have never had one of these go bad, but that could be it. There is enough steel line after that hose, that should allow the fluid to back off. Another thing that I can think of is the auto adjusters are installed wrong causing them to tighten up.

[wyattglock]

Thanks for all the input. The master cylinder is a brand new Delco unit. I geuss I should have stated that the drums are not new. But checking the finish seems like a good place to start. I may just go ahead and replace the rubber line. Also, the rear proportioning valve is original. I have never seen one of those in a parts listing, but maybe I can scrounge one somewhere.
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[donut]

Adjustment too tight?
I had a similar problem with a Furd Ranger ('94), I used to have to drag the brakes when I first started rolling, otherwise they'd lock up the rear. Once warmed up, they were fine. Switching from a full metallic shoe to a semi-met seemed to cure that.
Seems kind of odd, but that's what worked for me after I changed everything else.

[chrismoore701]

I had a problem like this on my dads van, be sure you have everything installed right, look at pics. I ended up removed the emergency brake bar, the flat bar that goes from the front show to the back, I think I had a spring on wrong, and just removed it. good luck

[75ChevyGuy]

Happens to me all the time. Normally after rain. I think it's a slight build up of rust, because after 2-3 stops it's fine again.

[tucsonjwt]

For me this was wet shoes/drums - driving through a large puddle, or fluid leaking on the drums - wheel seals leaking on my floating axle 3/4 ton, or could be a leaking wheel cylinder.

[sumran]

Originally Posted by 75ChevyGuy

Happens to me all the time. Normally after rain. I think it's a slight build up of rust, because after 2-3 stops it's fine again.

Common for me as well. It is so humid here it doesn't even need to rain. After the first stop sign it is not an issue. Sound like the original post is describing a more extreme case.

[wyattglock]

I do notice it more after rain or on a more humid day. For a reference on how hard it locks up, my particular truck weighs 6200 lbs, weighed at work on the scales for the semis. If I am going 10 mph, the truck will literally throw me forward fast enough to lock the seat belt and then stop the truck in about 3 parking spots. That is width not length. But after 5 or 6 times the problem goes away.
I have noticed that if I ride the brakes backing out of my driveway in the morning, it will try to lock up. The truck can push through it with a little help from gravity. Then when I go forward it will happen one time and go away. I also noticed that the pedal is very soft at first, but once the problem goes away, the pedal firms up. It might have about 2 inches of travel firm and almost touch the floor when it is soft.
I am taking tomorrow off from all the crap, going to go fishing. Then I will dive into it and see what I find.

[Desert1957]

Originally Posted by desert1957

i know this will sound stupid but check the surface of the drums. My truck when cold would do the same thing , but after having the mirror finsh (glazing) machined off the drums it was fine.

Desert

^^^^^^^^^^^^^^^^this^^^^^^^^^^^^^^^^^

[cr34t3d_by_d4rkd3n]

my truck does this after i set the parking brake and when i wash it. have u rebult the drums? if not a return and self adj kit may help. rebuilding mine this weekend. so ill let ya know hahah. also yes. a slightly leaking wheel cyl will cause it to b wet (greasy) and cause them to unexpectingly lock

[wyattglock]

So i finally made some headway on this problem. I found that the hard line from the prop valve to the rear axle had a split in it next to the passenger side header. With the power bleeder it didn't produce enough pressure to duplicate the problem, and after the pressure bled off the drip stopped also. However, when I pushed the brake pedal it would shoot fluid on the header. Lovely smell! So yesterday I ran a whole new brake line from the front prop valve to the rear axle. Re bled everything and pulled the drums to inspect those. They look pretty good, not to smooth, but not cut up with grooves either. Thanks to all for the suggestions!

[donut]

Good to hear. Living up north, I used to visually inspect brake lines etc. every fall.

[wyattglock]

That is a good practice to get into. I don't know how well they work out, but I figure if I have trouble with the line again I will try the stainless steel lines and see if that takes care of it.
On a good note, the rear brakes dont drag and lock like they used to.

[Daytona#3895]

This is a common problem on the Jeep Wrangler. Where the rear shoes ride on the backing plate gets rusty real quick. The first few applications the shoes expand against the drums and the return springs won't release completely because of the rust build up. After a bit they will losen up. I usually sand the contact points on the backing plate and use just a small dab of grease there. Not to much or it will fall off into the drum and contaminate the shoe. I know its not a Wrangler, but its something to look at.

[wyattglock]

Thanks for the tip. Even after all this heartache with the rear drums, I am thinking about converting to disc brakes. DIY4X makes a bolt on set of brackets, and I can get everything else locally. So for now, I will just keep trying things if I have more problems. Definitely got a lot of good info from the responses here!
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[1LowToy]

I spoke with a guy that literally has done thousands of brakes on chevys. They had the same experience. He found that if he bevels the leading edge of each brake shoe the problem is gone.