Brake woes

[The_Josh_Bear]

Hey guys I haven't been around in a while, but I have a significant brake problem I need help with.

Original problem: rear tires lock up SUPER easily under braking, seems like fronts aren't doing much.
By super easily I mean in my 25mph neighborhood if I'm not paying REALLY close attention AND braking while in gear one of my rear tires will lock up and just drag.

Important notes:
No leaks at any external part.
Wheel cylinders are not leaking.
New shoes installed recently.
Parking brake works great, shoes are adjusted fine.
RABS valve spring removed years ago and fuse pulled. Worked fine all this time and wasn't changed until troubleshooting the issue.
Calipers and pads are new last year, plenty of meat left on them.
Before testing the brakes all 4 corners were bled plus the RABS in this order: RR, LR, RABS, RF, LF.

Things I've done since original problem:
Since my fronts seem to be barely doing anything, I suspected a bulging hose and replaced them both. No change. (they weren't looking good anyway)
Pulled and cleaned out the RABS valve, hoping it would be the problem.

Now the pedal just sinks to the floor. Bled brakes more and then some more, same problem.

******Why oh why don't the front brakes engage even if the RABS is dumping?????? Aren't the two pistons in the master cylinder separate hydraulic circuits???********

Because I bled the system properly I figured the MS was giving up so:
Last night I changed the master cylinder, brand new unit from NAPA. I bench bled it! I installed it, measuring the pushrod and adjusting. I realized after 14 years that the original was too long but yet I never had brake drag issues...dunno what's up with that. Bled brakes as above.

Current problem:
Pedal falls to the floor with engine on, and not real slowly. Rear brakes engage and lock up but less aggressively than before. It's at least sort of derivable now. Today I'll put together a RABS bybass to remove the valve from the equation. I still can't get over not having front brakes if the rears are all screwed up. That sounds like the worst idea ever.

Edit: could this have anything to do with the proportioning valve for the front brakes on the MC? I re-used it when I swapped the MC's. I don't know how they work or if they can break, what makes them tick, etc. Pretty much nothing online about them.

Thanks guys!
Joshua

 

[Thewespaul]

What front pads are you using? Is it possible the shoes got mixed up- long side flipped with short side would cause lockup’s like that.

 

[franklin2]

I think I have heard of that happening too. If both long shoes get on one side, and both short ones get on the other side, the side with the long shoes will lock up.

But regardless, you have added to your problems. The pedal falling to the floor is not normal. And it didn't do that in the beginning. Been there done that. Breaking stuff as I am trying to fix stuff.

Your pedal will be low if you have air in the system, and/or your rear shoes are not adjusted out enough. Have you been backing the rear shoes away from the drums hoping to keep them from locking up? That will cause a low pedal.

I have had my RABS valve bypassed for years. The line goes directly from the master cylinder to the rear brakes. Front line goes directly to the tee and then the fronts. It will lock up the rear brakes during a panic stop when I really bear down on them, but that is to be expected with a pickup. Normal braking it works fine.

 

[The_Josh_Bear]

What front pads are you using? Is it possible the shoes got mixed up- long side flipped with short side would cause lockup’s like that.

Front pads are something I got off of Rock Auto I think, dunno the brand. Pretty sure they are fleet rated semi-metallic. Definitely not organic or ceramic.

I'll have to double-check I got them right but I must admit I never knew they were "short" and "long" until the last time I did shoes...doh. A quick google search says the long side goes towards the rear and short shoe faces forwards, right?

I think I have heard of that happening too. If both long shoes get on one side, and both short ones get on the other side, the side with the long shoes will lock up.

But regardless, you have added to your problems. The pedal falling to the floor is not normal. And it didn't do that in the beginning. Been there done that. Breaking stuff as I am trying to fix stuff.

Your pedal will be low if you have air in the system, and/or your rear shoes are not adjusted out enough. Have you been backing the rear shoes away from the drums hoping to keep them from locking up? That will cause a low pedal.

I have had my RABS valve bypassed for years. The line goes directly from the master cylinder to the rear brakes. Front line goes directly to the tee and then the fronts. It will lock up the rear brakes during a panic stop when I really bear down on them, but that is to be expected with a pickup. Normal braking it works fine.

I actually did put both longs on the same side and shorts on the other when I replaced my axle 2 years ago! Lame. (I recently did both axle seals and replaced the shoes again, correctly!)
Overall it was fine, I guess the one tire tended to skid more than the other but not really a big difference.

I haven't backed off the rears except a little bit early on, they are within spec. The parking brake works great, which it would not if they were backed out too far.


Anybody got the answer as to why both front and rear brakes don't work if the rear has air/leaks? I thought they were separate hydraulic systems and therefore for safety's sake one set of brakes should still work if they other set has a major failure.

 

[Thewespaul]

Do you have brake dust accumulating on your front wheels or does it act like they don’t do anything? The rear drums in the shop truck I rebuilt a long time ago and work excellent, better than the front did so I upgraded to the slotted rotors and ceramic pads from stoptech. Enormous difference in stopping and it’s very even between the axles.

 

[The_Josh_Bear]

Do you have brake dust accumulating on your front wheels or does it act like they don’t do anything? The rear drums in the shop truck I rebuilt a long time ago and work excellent, better than the front did so I upgraded to the slotted rotors and ceramic pads from stoptech. Enormous difference in stopping and it’s very even between the axles.

Yes they *were* working ok, not amazing but that could be something else contributing. Then the rears started locking up easily and in about one day went from "fine" to where I almost went totally sideways through an intersection in the rain trying to take a left turn on a green arrow.
I grabbed some Centric slotted rotors in 2016 and they helped a lot, same pads are up front as when I put them on.

 

[The_Josh_Bear]

Double-checked and the shoes are on correctly and not oil contaminated.

 

[Thewespaul]

Maybe you got some crappy calipers?

 

[The_Josh_Bear]

Do we have step bore master cylinders?

 

[The_Josh_Bear]

Maybe you got some crappy calipers?

Actually that's a thought... the right side drags constantly. I took it apart and cleaned up anything I could find, which wasn't much and for a few pedal presses it was great! Then back to dragging. So I'll need at least one more caliper anyway. Gar.

Can calipers bypass internally? I don't think so since they are just a cavity that fills with fluid. Fluid has to go somewhere, eh?

I think the conclusion is bad MC out of the box, rears work great and fronts suck terribly. However that's the exact problem I had before the MC change so I'm hesitant to condemn this MC. Been there done that, too.

 

[Thewespaul]

I would change then both, could be that the fit of the pistons themselves is the issue which causes them to stick on the one side and resist freely sliding out to contact the pad. These vacuum assist brakes and stock master cylinders actually don’t run much line pressure for such heavy trucks, they are pretty sensitive to bleeding issues and bad wheel cylinders/calipers.

 

[chillman88]

Are your calipers coated? We had an issue with the coating causing too much interference at the slide. We had to grind the coating off at the ends and then they were fine and stopped dragging. Food for thought.

 

[The_Josh_Bear]

Are your calipers coated? We had an issue with the coating causing too much interference at the slide. We had to grind the coating off at the ends and then they were fine and stopped dragging. Food for thought.

Mine are coated! Good thought, it's chipped away some but overall intact. Might be why that right side is sticking. Thanks I'll chip it off and test it out when I can.

 

[The_Josh_Bear]

I went and bench bled the old MC and reinstalled, same pedal but not as bad, it seems. Hard to tell.

 

[junk]

My thought is to try bleeding the MC really well. I've done a poor job bleeding the MC before install and had a heck of a time getting brakes on some vehicles.