Rear brakes grab

[TexasRed]

Well, it's been awhile.

I'm having an issue with the rear drum brakes. It's a 1984 GMC C-10, JB3 brakes

I'll try to tell the story so there's a good idea.

About 4 years ago, I had:

new drums,
new shoes,
rebuilt front calipers,
new rotors,
new pads,
new wheel cylinders (centric),
adjuster kits,
new hardware for the drums,
New hoses all-around.

Braking was phenomenal.

About a year ago, the rears seemed to occasionally grab something fierce trying to get out of the neighbourhood at slow speeds. Cleaned out a bunch of brake dust, mechanic tried to say that it was moisture but it was doing it after a dry day or two. Sometimes it would rain and wouldn't do it. Bled the brakes. Anyway, changed from Centric HD shoes to some Bendix. That seemed to help quite a bit, but then it started doing it again. Maybe while the shoes were conforming to the drum, I wasn't getting aggressive actions at first. At some point, I had taken a 10 min trip on the freeway, pulled off to surface streets and coming up to a light, I was giving it a little more than just a slow stop, no where near a panic brake, just a little extra. The rear wheels grabbed like I was panic braking. I let off the gas and then touched the brakes again. Since I was coming up to a red, I just kept my foot and let it slide. Then after the light, tried to make it do it about a quarter mile down the road, it acted normal.

I've replaced now:

the master cylinder to a Raybestos aluminum one.
Proportioning valve the 14041636 from Pirate Jack. I pulled the metering valve and cleaned the little piece of plastic off the molded seal that they leave on there. I think that's what causes some of the leaks people get.
Parking brake cables. They rusted on me. I was hoping it was just an adjustment, but then they locked up tight and ended up smoking those Bendix shoes pretty quick.
Using Wagner DOT 5.1 fluid (it's been flushed through at least once to try to catch contaminates)

The grab seems to be fairly even. I think it felt like a little bit of a pull to the passenger side (right side of the truck), but the tire pressure was 5 or 6 psi on that tire. The drums were machined because of the parking brake sticking. One side was really bad and required 3 passes to clean up. I ran some new centric HD brake shoes on the drums pre-machining but not enough to bed them in. After getting them bedded in, it's the same thing. Pulling up to a light and putting a little more than slight pressure on the brake pedal and the rear skids.

I've seen the posts regarding problems with JB3 to JB5 conversion problems, but everything seems to be the correct JB3 system. I've got the single diaphragm booster. It looks original. It seems to hold vacuum but I'm not sure if there's another way to test some of the valving inside to make sure it's not giving extra boost. The brakes release when you get off the pedal.

I bought another hose for the rear brakes but the "custom" one a company sent me leaked at the crimp and it appeared to be a sunsong one. Oh well. But I could blow air through the old hose so I don't think there's a restriction in there.

It could still be the proportioning valve, but I don't think putting an adjustable on there is a very good fix. Nor is it to put weight in the bed.

There is some rear noise now when I apply the brakes, and it could be too much pressure. Also, one of the things pushing me to think it could be the booster, is that at idle with the truck in gear, sometimes it idles smooth and sometimes it idles significantly rougher. Could be the quickfuel carb. I could add a few more RPMs. I'm running a summit racing ignition box, but I could convert back to the module for a little bit to set the RPMs via tach. However, I can pull the booster check valve after the engine has been shutoff for awhile and get the "whoosh" so it's holding some vacuum.

Any thoughts?

[philo_beddoe]

The rears on my 86 s-10 we’re sticking horribly. It was the e-brake. But before I fixed it, timing chain busted, donated the truck and they drug it away, literally skidding and screeching as they towed lt along.

[VileZambonie]

Sounds like you have an unbalanced braking system. Why are you running DOT 5.1 when you should be running DOT 3? If you are using the JB3 setup then run a factory combination valve. Post pics of both sides if you want us to quality check the installation of the components

[Mike81K10]

I have heard that DOT 5.1 should not be used unless the whole brake system was replaced, new system. Flushing does not clean the system of Dot 3 or 4. Could have gone with Dot 4 and no flushing would be needed.

[TexasRed]

Vile, I was running the factory one but the rears were grabbing, so after eliminating everything else, it should have been the proportioning valve. I can probably get my old one rebuilt. This brass one is super common. It's the disc/drum universal one, but I'm not opposed to going back. I haven't seen too many factory new ones for sale.

The DOT 5.1 is supposed to be fully compatible with DOT 3 and DOT 4, just a higher boiling point. It's DOT 5 that's not compatible with DOT 3, 4, or 5.1. I wanted something with a pretty high boiling point so if it sat for awhile, any water wouldn't lower it too bad. I used to travel for work and the plan was to be gone for some stretches. I can reflush with 3 or 4.

I will try to get some pics up shortly of both sides and different possibilities. I'm hesitant to believe it's this because it doesn't pull to one side, both sides grab equally. However, I want to find the issue.

Thank you for the replies and ideas.

[TexasRed]

Passenger (right side) first.

[TexasRed]

Driver's side. No dog on this one.

[MIKE S]

Something doesn't look right on the first two photos of the right side. Lets see a better picture of the side of the wheel cylinder on the left and the emergency equalizer rod on the left at shoe contact point.

[TexasRed]

see if this works

[TexasRed]

May have found the issue. The front pads seemed to be glazed over. Plenty of material left. Should I try sanding it down to get rid of the glaze or should I go ahead and just get some ceramics?

[MIKE S]

Ok rear looks good. From the first picture angle I wasn’t sure that the wheel cylinder rod may not have been engaged. Front brake deglaze may help but I have had an issue with a Ford booster once that would throw you thru the windshield with light apply. At the time I didn’t think booster as it felt fine testing it at rest. No vacuum hard. Start vehicle and pedal felt assisted as normal. Proceeded to replace every single part on truck except booster. Booster fixed it. For peace of mind I would probably replace booster.

[TexasRed]

Well, I spoke too soon. So I got to replacing the brake pads with ceramics and drove it some. I don't drive the truck often, however I did try to bed these in. At first they were a lot better but I eventually pulled them because of the same symptoms and they too looked glazed.

Since I didn't really see a big difference between the side to side in braking so I figured maybe the rotors had some kind of heat treatment or the bedding in process glazed the pads. So I replaced the pads and rotors. I used some bosch quiet ceramic pads. But it's pretty loud right now and the rears still like to grab really quick and it feels like it's dragging a little.

I've gone ahead and ordered NEW calipers from rock auto and new hoses. The calipers are uncoated, I may try some caliper paint. Anyone have any suggestions?

I would have thought it would be something that affected both wheels not potentially a problem with two wheels so when the calipers come in and I get the old ones out, I'll try to update.

[Mike81K10]

Brake caliper ceramic paint, comes in different colors.

[bedwards]

after further study, my post was wrong  sorry don't know how to delete

[dusterdude]

My 85 does that when the brakes are cold,a little bit of heat and they stop grabbing

Sent from my LM-X420 using Tapatalk