Brakes: someone please explain this to me

OK, $ sink has 11" drums on the front, 10" on the rear. the fronts are complete, only one of the rears has self adjusting hardware. I installed the 11s about 6 months ago and was happy with the way it stopped, straight and front lockup berfore rear.


I moved,

now I do not have a garage to keep the jeep in so it sits under a tarp and gets driven ocasionally. 2 weeks ago I go to drive it and it tries to do a circle when I brake. turns out the right rear is locking up before the others. it happens to be the one without the self adjustment. I went through the fronts again, new wheel cyls, made sure the hardware was all installed correctly. shoes apear slightly glazed but clean.

no change

checked the left rear, adjusting hardware all present and working correctly, shoes move when pedal applied, shoes worn but good. can't turn drum when pedal applied.

no change.

final try, adjust right rear (non self adjusting) until it barely contacts the drum with pedal applied. drive, feels ok but not great at least the RR is not locking.

couple of laps of the neighborhood working the brakes, bleed again, drives ok but if I pump and then hit hard..... RR locks up. :? :evil:

please help, I am ruining a good set of tires and my wife's patience...

 

i had the same problem on my 87 gmc surburban... the drum was warped just slightly worked fine with little pressure but when i sank my foot into it it locked it locked up real bad.. you have any vibration when you apply just a little break ?

josh

 

I don't think it is the drum, I adjusted this brake to where it should have no braking effect at all, and it still locks up before the other 3, wondering if my MC is shot....

 

no kinks, all brakes are expanding when pedal is applied

 

Randy-

I had to replace my wheel cylinders when my 10's were locking up. Cheap fix... and if you haven't yet, I'd have the drums turned just to be safe

 

had to pump because I just replaced the front cyls to try to fix it this morning and assume I stll have a little air in them. bled them twice, got bubbles both times. bled other rear, no air.

 

You might want to drop the drum and check the linings on the right side. A contaminated lining will pull while a good one won't.

 

M/C has no way of telling the left from the right side of your Jeep.

Check the drums/linings as stated above.

 

the rear brakes are different, one has adjuster while other does not, I am really not worried about them not acting the same yet, I am more concerned with why the rear is locking before the fronts.

 

I was going to reply to this ... then didn't ... but noone has brought this up. IMO it's bad practice to use dissimilar brakes on the left rear v. the right. Ideally they should work the same, but obviously something is not working right. I wonder if you have mismatched cylinders or there's something wrong with the self adjuster or...

The rears will lock up before the fronts if the same force is applied to the front and rear. Basic Newton ... f=ma ... the acceleration front and rear has to be the same, and the mass on the front wheels is way higher due to weight transfer. Usually this is handled by smaller diameter cylinders on the rears, so that the majority of force is applied to the front tires. It seems that a proportioning valve is an after-the-fact effort to fix such a problem.

 

Adjusting bakes adjust. When you back up and hit the brakes they loosen up a bit....then as you brake in the forward direction they tighten back down automatically adjusting themselves. As stated in an earlier post not a good idea to have one non adjusting and the other adjusting. To fix the prob...change the right to adjusting or the left to non adjusting....unfortunately you will continue to have this problem if you don't.

 

Just to clarify- self adjusters only adjust themselves one direction- tighter.

They don't adjust both directions.

 

Another two cents.
The flex brake lines that connect the wheel cylinder to the solid brake line on the axle can cause problems. They can internally fail. The inner lining can develope a pinhole leak, which causes fluid to become traped between the inner and outer casings. This area between layers slowly fills and colapses the inner tube to the point where reverse flow is restricted. Constant pressure is applied due to the outer tube being softer and more flexable than the inner. This traped pressure and colapsed inner tube allows brake fluid from the pedal pressure
to get the the slave cylinder but the normal return force of the brake shoe return springs isn't enough to pull the shoes off the drums. The failure is internal to the brake line, the lines have multiple layers of construction and this type of problem is not always detectable form the outside. You have adjusted the brakes bled them and chased everything else down. Short of the solid line being crushed between the flex line and the tee block where the rear axle lines join atop the differential this seems to be the most likely culprit.
If one is bad change them all.
Good luck

 

Also check the rear wheel cylinders and make sure you don't have a situation where one piston in the cylinder is frozen. If so, all the hydraulic force going to that cylinder will act on one piston and cause a wheel to like up like you're describing. If you have dissimilar brakes on the rear it's also possible that you have wheel cylinders with different diameters which can cause problems like you're describing. Nickmil.

 

Self adjusting brakes adjust themselves when you back up and hit them harder than normal, they do not adjust when going forward and they never loosen lightly in reverse unless there is something wrong.
I'll second Nicks idea about dissimillar wheel cylinders in the rear.

 

Ok to clear the water a bit (also to eliminate some tendancy to think of me as an idiot):

1. I have checked both pistons of both rear wheel cylinders, they are free and move out as they should when pressure is applied. fronts have new cylinders and the shoes show no sign of contamination.

2. I mentioned the lack of adjustment hardware to make sure it was known up front. I adjusted both rear brakes the same (manually) prior to testing to eliminate the adjuster coming into play. remember it is the one without adjuster that is locking up.

3. I have checked for leakage in all 4 brakes and other that some fluid from the front wheel cylinders (that was trapped in the end seals), there is none present. Front hubs had new seals installed 8 months ago.

4. Both rear cylinders appear to be 1" bore while the fronts are 1 1/8"

5. after initial testing with rear brakes adjusted evenly, I adjusted the one without hardware until it should have had no braking effect at all, thinking this would remove it from the equasion. later I realized that the
pistons would move out until the shoes met the drum no mater where I had it adjusted to, then apply both rear brakes evenly (in theory).

I cannot work on it again until this weekend. I have aquired a new set of front and rear shoes, rear wheel cylinders and master cylinder (oh and a large supply of fluid). I intend on systematicaly eliminating each component. Thank you for all of your ideas, I was counting on the input to ensure that I had not missed anything along the way.