Rear brakes locking up

[jlolson]

Before I spend anymore money I thought I'd ask... There are several goood posts on locking rear brakes but I haven't seen my exact scenario.

Couple monts back I spotted fluid coming from my rear passenger side drum (71 C20 w/rear leaf). Pulled the drum and the axel seal and wheel cylinder were shot. Stripped and replaced all hardware on both sides including new shoes, wheel cylinders, and axel seals. I was meticulous about cleaning up the grease mess from the seal failure.

Trucks back together, pedal feels great, brakes work fine. After about 10 minutes of driving I start pulling right when braking, couple more pedal pushes and the back passenger side starts locking up. Take it home and let it sit awhile it's fine again... Until 10 mins of driving then it starts pulling, then it locks up. Keep driving and smoke.

I read forums, got some advice. Played around with rear brake adjustment multiple times and issue repeats. Made sure adjusters weren't backwards, re-cleaned rear brakes multiple times in case of remaining axle grease, replaced flex lines, and hard lines at axle. Still has the issue.

The rear cylinders seem to be building up pressure over time then not releasing until the truck sits for awhile. My old man says replace the master cylinder but before I spend anymore money I wanted to see if anyone has any ideas on this one.

Thanks in advance.
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[davepl]

Residual grease or oil would be my first guess, but sounds like you're fairly sure you've got it all. The rear brakes are on the same line, they just T off after the rubber line, so I would think anything related to the master would affect both sides equally. I was wondering about a residual pressure valve, but again that should affect both sides.

Did you replace the shoes -after- everything was finally clean? If the pads ever got grease or oil on them, no amount of cleaning will likely get it all.

It really sounds like a thermal expandsion issue, given it's OK when cold. If it were me, I'd back the adjuster off on the offending wheel and see if that helps, and if not, I'd swap shoes side to side to see if follows the shoes.

[68gmsee]

This is sorta lengthy explanation:

Exact same thing happened to me about 15 plus years ago. I went through the whole trouble shooting thing and replacing parts and it still would lock up just as yours is doing.

In my case it was the rear rubber brake hose on top of the rear axle to the frame. When I took it off I found it was completely closed inside. I couldn't blow any air through it.

Now here is what happens. The hose collapses internally and brake master cylinder pressure will force fluid through but the spring return pressure is not enough to retract the brake shoes so they lock up. To test you can jack up the rear wheel, start the truck and have someone pump the brakes. When they lock up, loosen the bleed valve and see if that unlocks them.

If that unlocks them, then more than likely it's the rubber brake hose. If it doesn't, then you're talking about shoe contamination, brake cylinder failure or other mechanical problem with the brakes.

[gmc 2500 4x4]

prop valve near master cyl , collapsed hose is a good one. cant tell by looking at it

[slikside]

subscribed

[mtm_seven]

+1 for collapsed hose. Same thing caused ghosty issues on one of my trucks.

[jlolson]

"flex lines theory"

So I did replace the flex line and the hard lines on the rear axle. That bridge has already been crossed. Zero change in the symptoms I described. FYI the LMC pre-bent rear brake lines are worthless, if they were designed to fit my truck you could have fooled me. They don't look anything like what I removed, were over 1.5 feet too long, and had weird useless bends. The tee fitting was equivalently useless as it has no mounting hole and doesn't match up with the stock bracket

"Axle Grease Theory"

Although I did clean the rear drum thoroughly and I have re-cleaned them multiple times the advice to switch the shoes to the other side is a good idea which I think I'll try. The rear drum was a mess and if it is truly that sensitive to residual grease I'll give it a try.




I should mention that I'm not convinced it's just the one side. The pedal seems hard as a rock ever since I did the brakes and since I can only drive the thing a very short distance before it starts locking up the rear it may just be initial adjustment that is responsible for only one side locking up. Either way switching shoes from side-to-side should help diagnose this issue.

[LEEVON]

I beat my head against the wall for months after a brake job. Driver's rear would lock up for no apparent reason all the time. I pulled everything, inspected, cleaned, replaced, compared, cussed... Eventually it occurred to me that I might have swapped the drums after having them turned. The clearances / wear patterns were slightly different side-to-side and the drum was hanging up. When I swapped them back it was like a new truck

[cleszkie]

I had similar issues. The rear brakes would lock up on occaision for no apparent reason. The problem - I had installed the brake shoes improperly. Each wheel has two shoes, one with a shorter length of brake lining. Its been a while since I fooled around with drum brakes since I swapped in rear discs, but I think the short side of short shoe should face forward. Someone here will correct me if I'm wrong. Good luck.

[gmc 2500 4x4]

Originally Posted by jlolson

"flex lines theory"

So I did replace the flex line and the hard lines on the rear axle. That bridge has already been crossed. Zero change in the symptoms I described. FYI the LMC pre-bent rear brake lines are worthless, if they were designed to fit my truck you could have fooled me. They don't look anything like what I removed, were over 1.5 feet too long, and had weird useless bends. The tee fitting was equivalently useless as it has no mounting hole and doesn't match up with the stock bracket

"Axle Grease Theory"

Although I did clean the rear drum thoroughly and I have re-cleaned them multiple times the advice to switch the shoes to the other side is a good idea which I think I'll try. The rear drum was a mess and if it is truly that sensitive to residual grease I'll give it a try.




I should mention that I'm not convinced it's just the one side. The pedal seems hard as a rock ever since I did the brakes and since I can only drive the thing a very short distance before it starts locking up the rear it may just be initial adjustment that is responsible for only one side locking up. Either way switching shoes from side-to-side should help diagnose this issue.

inline tube has nice brake lines

[DT1]

I had the same issue with LMC lines. Not just break lines but power steering and A/C lines. The power steering line was so bad I had to cut it and install a Swagelok coupling just so it would clock right. I just returned the A/c Lines.

Originally Posted by jlolson

"flex lines theory"

So I did replace the flex line and the hard lines on the rear axle. That bridge has already been crossed. Zero change in the symptoms I described. FYI the LMC pre-bent rear brake lines are worthless, if they were designed to fit my truck you could have fooled me. They don't look anything like what I removed, were over 1.5 feet too long, and had weird useless bends. The tee fitting was equivalently useless as it has no mounting hole and doesn't match up with the stock bracket

"Axle Grease Theory"

Although I did clean the rear drum thoroughly and I have re-cleaned them multiple times the advice to switch the shoes to the other side is a good idea which I think I'll try. The rear drum was a mess and if it is truly that sensitive to residual grease I'll give it a try.




I should mention that I'm not convinced it's just the one side. The pedal seems hard as a rock ever since I did the brakes and since I can only drive the thing a very short distance before it starts locking up the rear it may just be initial adjustment that is responsible for only one side locking up. Either way switching shoes from side-to-side should help diagnose this issue.

[GuyO]

Umm, if not hoses and shoes then it sounds like a proportioning valve not centering correctly forcing the rear to lock and the front to pull. If it were me I would go back to bleeding with new fluid. It sounds like you are very methodical and understand brakes and how important it is that the hoses must be lower than the bleeder so wire them down and let the air raise up. The prop valve must be clamped and the rubber button held in place (bleed the rear drivers first, then rear passeneger, then front passenger, then front driver). With the system free of debris, clean fluid and a centered prop valve you have elimated almost everything except for replacing the prop valve. Good luck. G.

[jlolson]

Hello All - It's December and my truck is fixed. Thought I'd update this post.

Just a reminder - Axle seals went out on my C20, while fixing this I decided to do a full rear brake job including wheel cylinders, springs, hardware, shoes, etc. Only think I didn't do was adjusters because I still to this day can't find adjusters for a 3/4 ton with rear w/leaf. I could drive the truck for about 10 minutes before the rear tires would start locking up on braking, usually passenger side.

This isn’t my primary driver and it has sat quite a bit with no time for me to work on it. When I did have time I followed all the advice I could find on forums. The laundry list:
• Replaced flex line
• Cleaned the snot out of everything to make sure I didn’t have residual axle grease
• Swapped everything from side-to-side
• Resurfaced drums
• Replaced rear hardline
• Tried 75 different adjustments
• Flushed with denatured alcohol from front to back then purged with two bottles of dot3, made sure no air or debris in the lines
• Adjusted and removed e-brake to make sure it wasn’t dragging
Finally gave up on the rear (obviously suspect cause that’s where I did the work). Started shot gunning elsewhere
• Adjusted brake pedal
• Replaced proportioning valve
• Replaced master cylinder
• Checked vacuum booster
None of this worked. Only choice now was to drive it off a cliff or take it to the shop. First shop looked at everything, said it all looked good, then sent me packing. Second shop I found had a Chevy truck guy. He worked on this thing for about 2 days, went over everything I did, checked all the new parts, even put some of the old parts back on to make sure none of the new parts were causing the issue. After two days he asked for approval to replace the rear shoes, they were obviously new, but he said at that point that’s all he could think to do. He replaced them and the truck is golden. I put brand new NAPA shoes on that thing, never thought that they could be “bad” but for whatever reason they were expanding under heat. Been driving the truck for days now and it’s perfect. Still not sure how the shoes could be that faulty from the factory but I have no reason to disbelieve this as root cause since the truck is great now. Thanks brake shoe manufacturer! You cost me a lot of money and time!

[Longhorn 70]

Thank You jlolson. Now I have another suspect in my similar issue.

[Stocker]

That is a heck of a lot of effort to eventually pin the blame on bad new shoes. Glad it was you and not me.... also glad it's finally fixed. Did NAPA give a refund or replace the shoes? What shoes did you end up with? BTW thanks for updating this thread....

[jlolson]

Originally Posted by Stocker

That is a heck of a lot of effort to eventually pin the blame on bad new shoes. Glad it was you and not me.... also glad it's finally fixed. Did NAPA give a refund or replace the shoes? What shoes did you end up with? BTW thanks for updating this thread....

I didn't keep the box from the Napa shoes so haven't attempted to return them. For the record I've always found Napa to carry good parts and the counter staff is the best in my area. My guess is they would just offer an exchange for these and I won't be using Centric brake products again.

The new shoes are BrakeBest Select, I think it's an O'Reilly product.

[jd7]

I've been a mechanic for 20 years now experience has taught me well.Whenever you perform a service on a car or truck you tend to wear blinders.... meaning the new parts can't be defective,they are new. If a problem develops after the repair I look first at the repair itself. It sucks that you spent all that money only to find out it was a defective new part. I hate throwing rocks at companies but if a customer wants the less expensive part we use centric rotors & drums we must machine them before install or they will vibrate under heavy braking.

[scscomp]

I fought this problem for a long time before I found the solution. I had recently replaced the rear shoes with semi-metallic lined shoes. The rear brakes are designed to lightly scuff the drums when turning and the higher friction semi-metallic lining material was overheating and warping the drums. Even if you adjust the shoes to not scuff the drums, the shoes will self-adjust back to the scuffing position after you back-up a few times. The solution to my problem was to replace the rear shoes with organic lined shoes (probably what the original linings were). I turned both drums to remove the warp and the problem was resolved. Now to figure out what to do when they stop selling organic lined shoes.