Rear brakes locking

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mudpie

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Not lifted, not off road, but this is the only place that seemed obvious for a brake question.

So I've got a 1996 C2500HD, and now that fall is here I'm noticing a brake issue when the streets are wet. The rear is really touchy, and it wants to lock up easily. If I'm on a straight section of road, it feels like the anti-lock will kick in and release pressure, but I don't feel that I'm getting much in the front end. If I keep pressing the pedal harder, I start to get pressure in the front, but the whole system feels kind of weak, and I feel like I'm putting way more pressure on the pedal than should be needed to get it to stop.

Also, on occasion, if I'm going through the neighborhood and brake into a sharp turn on a side street, one wheel will lock up and it feels like the speed differential between the two wheels is locking up the rear end. I get tire scrub until I get going straight again and the rear unlocks.

I did a search, but most of the stuff I came up with was related to rear end swaps, meaning issues with master cylinders not being sized right for the wheel cylinders or similar issues. My truck is stock, and I've done nothing to the rear end since I got it a couple years ago. I also saw a few threads with similar issues that didn't get resolved, and a couple with suggestions of rear proportioning valve, but some of those were late '80's trucks. I don't know how similar the brake systems are.

Is there a common failure point? Logic tells me the proportioning valve, but I'm not really a Chevy guy, just a guy who happens to have a Chevy. I figure it's best to ask first. Any thoughts or suggestions appreciated.
 

RichLo

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Have you taken the drums off yet and actually looked inside?

a cracked drum, broken spring, or leaky axle seal could cause those issues also. but you'll never know until you actually look inside.
 

Bob L

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Rich Lo is spot on a leaky seal will cause grabby brakes when wet a broken part can cause brakes to lock intermittently if a part gets under the shoe. Look behind the tire ,is it shiney or wet from gear oil or brake fluid?
 

gnrl

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Do you have hydroboost? Several gmt400s with hydroboost I've had would lock the back wheels sometimes when you just tap the brakes. But for me this only happens in the first minute of driving from a cold start though.
 

Dahead76

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I was having the same issue on my C3500. Put new shoes, hardware and brake cylinder and it still would lockup. Ended up being my drums were slightly warped. Had them turned and no more issues.
 

cdub

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I had the same issue. I did not have an axle seal leak. I replaced drums, shoes, then wheel cylinders trying to get it to stop.it was greatly reduced after drum, hardware and shoe replacement, but still there.
Few weeks later It was suggested I check the baking plate, so took it apart again and where the shoes slide(rub in 3 places for each shoe I think) on the backing plate were grooves worn into the plate from the shoes. Instead of replacing the backing plate (recommended option), I sanded/filed those areas to make it smoother. Next rain I did not have the problem.

Not sure what combination of all those fixed it, or maybe just the shoes wore-in, but it is gone now.

FYI, I'm sure the recommended practice is to replace backing plates and not to file them down. Safer that way.
 

lxe4

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are these the 13" drums? my 96 has the 13s and they get grabby after a few months and a bunch of miles. about once a year or so i take them off and scub everything clean, relube the adjuster bolt and grind of the rust lip on the inner edge of the drum. makes a huge difference. I've had wheel seals go too and that certainly makes it grabby. I think it comes from having huge brakes and running around empty most of the time.
 

mudpie

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Other projects put this on the back burner. Who really needs brakes anyway, right?

I heard a little grinding noise yesterday when I was backing up and braking, so I figured it's time to check it out. I pulled both rear drums off and everything looked good. The shoes have at least 50%, all the springs and hardware is in place and seems to be functioning, and the drums have no scoring or cracks. Lots of brake dust, but it's all dry, so no leaking.

My fluid level is about half in the reservoir, and it looks like it's been leaking some around the front seal on the base of the reservoir, as well as around the cap. I'll try and attach a pic. I held the pedal down for a while and it didn't drop, so it doesn't seem to be losing pressure anywhere. I pulled the front wheels off and stuck a screwdriver between the inner pad and the piston and tried to compress the piston. It moved, but I had to put way more force into I than I would think should be necessary. Both sides were the same. Then I pumped the pedal a few times and there was still a gap between the inner pad and the piston. It seems the piston didn't really build up much pressure and return to a position of contact against the pad. My first thought was that both calipers are close to being seized. That would explain why it feels like my braking is all being done at the rear, and why my rear wants to lock up.

It seems odd to me to have both calipers bad though, but I suppose stranger things have happened. Before I go through the hassle and expense of doing the front brakes, I wanted to get some input on the likelihood of the calipers being bad vs. any chance my issue is with the ABS module, or maybe a proportioning valve. Is there a common failure point? And to answer a question above, I don't have hydro-boost.

Any input appreciated.

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phule

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Crack a bleeder in front & see if the piston goes back. If it does you may have a restricted rubber line. I live in a rust prone area so frozen calipers are nothing new up here.

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Tavi

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All of my issues have been with the rubber lines. But none of my vehicles have ever been to the rust belt.
 
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