whats wrong with my brakes????!!!!!

[roughedup72]

heres whats going on. whenever I am moving and apply the brakes the rears lock up. The fronts act as if they arent being applied at all. I have installed a new booster master cylinder and pads and shoes, also new rotors, calipers, shoes and wheel cylinders. The only thing that has not been changed is the proportioning valve. Everything was fine for a little while after insalling all the new parts but now its acting stupid again.

Also I was planning on using the setup off of the 93 burb that I just scrapped but am not sure what all i need to do to install the new booste,M/C, and proportioning valve. Any help would be appreciated. Thanks

[72lb4x4]

Do the brakes work really poorly, meaning are they very ineffective? The fronts actually do about 75% of the braking so it should be noticeable if they aren't working.

Bleed them?
If the fluid isn't new, flush and bleed?
Rears adjusted too tight?

[northerngmc]

Just a shot in the dark but any chance you have a leaking rear wheel seal? I understand that oil on the brakes will make them grab.

[leddzepp]

Originally Posted by northerngmc

Just a shot in the dark but any chance you have a leaking rear wheel seal? I understand that oil on the brakes will make them grab.

x 2

[Longhorn Man]

Another cause would be the rears adjusted too loose. Does the pedal go down further than you feel it should?
What happens if you pump the pedal up?
If the pedal rises on each pump, then sinks down when holding, you have air in the system.
if you pump them up and they stay high when holding, then your shoes need adjusted up.

[roughedup72]

The pedal stays firm if you pump them although I havent had to pump them it still locks up the rears rather quick. One time when I was driving it down this gravel hill I applied the brakes and the rears locks up but I kept rolling as if the fronts werent doing anything. Makes for a realy nervous situation. I havent checked the rear brake cylinders yet. I figured since they were new when I put htem on that they were still good but I guess they could go bad. I'm going outside to start the disassembly process and try bleeding them out to see if that changes anything.
Any thoughts on using the master cylinder and booster that I took off the 93 burb.

Thanks for the help guys.

[mike16]

when you bleed the brakes you need to reset the perportioning valve otherwise only one set of brakes will wokr

[Smallz72project]

I have never heard of resetting a proportioning valve in my life. The valves on these trucks...at least original valves just distribute the fluid predetermined on how they were made, if its bad it needs replaced. Is it possible that you got the primary and secondary shoes on backwards? I am a certified brake technician for Les Schwab Tire and alot of the time when I have a customer come in that has this scenario and just did brakes on their own have the shoes mixed around. The Leading (primary) shoe should be facing forward and you will notice the friction material is shorter in length then the trailing (secondary). Also check your emergency brake cable and actuating arm, sometimes they get stuck even if you arent using the e-brake.

As far as the swap to 93 parts, can be done. I would assume that you'll need the whole kit and caboodle though all the way to the pedal assembly and if you are using the original calipers and cylinders you can get an aftermarket adjustable prop. valve.

[fjthompsoniii]

brake dust built up in the drums will cause it as well.

[dwcsr]

Originally Posted by Smallz72project

I have never heard of resetting a proportioning valve in my life. The valves on these trucks...at least original valves just distribute the fluid predetermined on how they were made, if its bad it needs replaced.

They actually had a tool to keep the prop valve centered so you didn't have to reset it or recenter it. The valve is a dual purpose a meteing valve for the disc brakes and a safety valve so if a line broke or a cylinder went out it would slide that way and shut off the flow to either the front or rear depending on which one lost fluid. Keeping these valves centered is very important in proper bleeding of these old system. Every now and then a centering tool comes up on ebay but they no longer make them.

[292farmer]

could it have slid and stuck to one side sending only fluid to rear causing them only to work and since their the only ones their locking up trying to stop the truck? jw

[dwcsr]

Originally Posted by 292farmer

could it have slid and stuck to one side sending only fluid to rear causing them only to work and since their the only ones their locking up trying to stop the truck? jw

Yes. I had that exact thing happen on my brake job. Rears grabed and the front didn't work well. I made a tool to centered the valve then bled the brakes and it worked much better. I also had unseen contamination on the shoes making then grab, so I had to replace them again. See if you can use a brake gauge from the parts store and adjust the intial rear brakes with it and do the final by ear. The tighten till it doesn't turn and back off doesn't work well on this series trucks.

Once I centered the valve with the tool, bled them out again , replace the shoes again and adjusted them with the gauge all work as it should. The early disc cars and truck were sometimes a PIA to get stopping they way the later models do. KD makes the brake gauge for about $15

[mrein3]

Originally Posted by Smallz72project

I have never heard of resetting a proportioning valve in my life. The valves on these trucks...at least original valves just distribute the fluid predetermined on how they were made, if its bad it needs replaced. Is it possible that you got the primary and secondary shoes on backwards? I am a certified brake technician for Les Schwab Tire and alot of the time when I have a customer come in that has this scenario and just did brakes on their own have the shoes mixed around. The Leading (primary) shoe should be facing forward and you will notice the friction material is shorter in length then the trailing (secondary). Also check your emergency brake cable and actuating arm, sometimes they get stuck even if you arent using the e-brake.

As far as the swap to 93 parts, can be done. I would assume that you'll need the whole kit and caboodle though all the way to the pedal assembly and if you are using the original calipers and cylinders you can get an aftermarket adjustable prop. valve.

Originally Posted by Huck

THE FACTORY SHOP MANUAL TELLS YOU HOW, WHERE, AND WHY IT IS NECESSARY TO RESET THE PROPORTIONING VALVUE WHEN BLEEDING THE BRAKES. I replaced shoes, master cylinder, proportioning valve, wheel cylinders, etc. Problem would keep coming back (rear lockup). Replacing the brake lines and hoses fixed the problem.

This is EXACTLY why I do my own brake work and don't trust the shops.

When doing major surgery, like replacing front hoses, rear wheel cylinders, or a blown out steel line, you HAVE to hold the button down on the front of the proportioning valve. If you have 3 people, one guy can hold the botton, one guy can pump the brakes, and you can run around cracking bleeder screws. If you don't have that 3rd set of hands, you can simply make the "tool" to hold the botton. All you need is a piece of heavier sheet metal with a tab bent to hold the button and a hole drilled to bolt it to the valve.

Here in the rust belt, ALL leaky rear wheel cylinders, and some front brake hose jobs cost you a brake line job. You can NOT break the line free without twisting the steel line apart (on the rear cylinder - some time ask me how to get around this on the front). The replace rear wheel cylinder procedure usually goes like this:
1. Identify leaky cylinder.
2. Order part and grab a piece of line incase you twist apart what you got.
3. Remove cylinder by twisting and breaking line.
4. Find other end of line you just broke.
5. Cut it flush and use a deep socket to remove it from where it is.
6. Bend up line you grabbed in step 2.
7. Replace wheel cylinder.
8. Hold button on prop. valve so you're not stuck trying to re-center the prop. valve.

Been there, done that about 13 ka-zillion times and only once had to chase around the brake light. I believe I said the same thing when the old timer showed me the button, "I never heard of the button".

Back to the problem at hand - and the reason why I talked about replacing steel brake lines. If I had a 1972 that never had the brake fluid flushed, and I was having issues, I'd make that tool to hold the button, get a friend to help by pumping the pedal for me, and bleed all 4 corners until clear fluid came out of each bleeder screw.

Then I would adjust the rears properly after making sure the shoes are in the correct position.

A 1972 (disk brakes) with fresh brake fluid and properly adjusted rear drums will stop like a 2010 disk/drum truck. It has to.

Oh and check your brake light on the instrument cluster. It may be burnt out.

[roughedup72]

checked the rear brakes and as far as I can tell they were pretty loose. It looks like they havent even beeen used, very little brake dust. I adjusted them and rebled them along with the front and also bled the prop valve. Tried it out and same thing. Just moving it around in the driveway barely moving and hit the brakes the rears lock up.
Smallz72 i didnt check to see if the brake shoes for primary or secondary sides. I will check that tomorrow if its not raining. Thanks for the info.
As far as the set up off the 93, I have the booster,m.c. and prop valve. somebody on here said I would need to shorten the rod but thats all I've been told. I wish I knew of somebody who has done this so they could help a brother out.
Thanks again guys

[292farmer]

we have a 1996 3/4 ton when humid or wet/raining out the rears will lock up constantly and make a horrible screeching noise, once dry or nice out they act normal, i guessed the moisture was expanding the shoes causing them to catch quicker but we never figured it out and it still does it to this day (we just know to expect it now)

[mike16]

thats what I was tryin to say.. recenter the little plug in the perportioning valve. If you dont recenter it after yo blead each set of brakes only one set, front or rear will work. a hint that recentering is needed is the brake idiot lite stays on.

[aboveall]

i have 96 chevy thoe and had rear brake problems with the rear locking up when aplying brakes,so i disconected the eb cable and the problem was fixed never did it again so i examined the eb mechanism in the drums and got bent some how and interfering with the brakeing of the shoes i fixed it and everything works great never had a problem again,mayb e check your eb system

[Kayotic]

My '86 C10 does this when it sits awhile. Goes away after a short drive. I've assumed it was due to rust on the rear breaks, I never bothered to look into it...

The point that you said it doesn't feel like the front is stopping anything is intresting. My first guess would be the prop. valve.

[Huck]

THE FACTORY SHOP MANUAL TELLS YOU HOW, WHERE, AND WHY IT IS NECESSARY TO RESET THE PROPORTIONING VALVUE WHEN BLEEDING THE BRAKES. I replaced shoes, master cylinder, proportioning valve, wheel cylinders, etc. Problem would keep coming back (rear lockup). Replacing the brake lines and hoses fixed the problem.

[mbgmike]

You can still buy the tool. Most places have them including us. they are less than 5 dollars. I have installed a lot of brake systems and never had to bleed the Prop valve even on new systems. Not sure its absolutely needed but if there is problems I would bleed it.

[aboveall]

i never in my hole life ever had to adjust a proportioning valve yet,i do all sorts of custom work on my trucks with big lifts changing rears i put 2.5 military rears in my trucks tires from 44 to 66 on them and i still use stock proportioning vavles and never touched one yet i bleed my brakes and they always work perfectlly

[LONGHAIR]

Originally Posted by aboveall

i never in my hole life ever had to adjust a proportioning valve yet,i do all sorts of custom work on my trucks with big lifts changing rears i put 2.5 military rears in my trucks tires from 44 to 66 on them and i still use stock proportioning vavles and never touched one yet i bleed my brakes and they always work perfectlly

1966 is the magic year for this.
Before that time there was just a single line coming out of the master cylinder that fed into a "distribution block". All of the brakes worked from that single source.
Starting in '67 Federal mandates changed everything to "dual reservoir" master cylinders, which seperated the front brakes from the rear. This is what brought out the need for the porportioning valve that could handle it.
Keeping the valve centered is all about "balance", which is how it "shuts off" one end or the other.

[roughedup72]

Thanks for all the input guys, I really appreciate it. I went back out this morning and took the wheels off and noticed that the drivers side shoes were on wrong (primary side and secondary side) Put them on the right way and cleaned everything up real good. Also went ahead and changed the gear oil in the rear seeing as how its been a long time since it has been changed. I'm heading out to test drive it and see how she does.

Thanks again

[Smallz72project]

Good to hear that you found something and were able to correct it, its always frustrating when you go through something a million times to only still be scratching your head. Let us know how it turns out and pay attention to how you think the front and rear are interacting with one another. As many on here have stated you may have a "unbalanced" prop valve. That you could try and rebalance see if it helps but if it was me, and in my professional opinion, when it comes down to something as vital as your brakes, I would just recommend replacing the prop valve with a new one. Cpp has some really nice adj. ones. Best of luck and keep us updated.

[roughedup72]

Test drove it up and down the road, got up to speed and hit the brakes and everything worked good. I also didn't have that annoying squeal when backing out the driveway. So I drove it down this lightly travelled road sland stood on the brakes, They didnt lock the tires untill right at the very end. Put it in reverse gave it the gas and applied the brakes pretty hard and the fronts did lock up but not untill the end which sounds about right. I also tested it on the gravel hill that it slid down and everything worked the way it should. I may end up buying a new prop valve anyway just for security,
Thanks again for all the help.