Left front brake locks up and releases after pumping pedal lightly?

[RichLo]

Brake bleeding can be very aggravating

You got clean fluid out of every bleeder?

Yes, the brakes are bled correctly with the 2-person bleed. I have bled hundreds of brakes via 2-person, vacuum, and gravity but IMO 2-person is the best. Since he is selling the truck I want to know I'm doing the best for the new owner and like I said a few minutes ago I am 100% confident all 4 corners are absolutely perfectly bled now.

Looking for advice on what else could be wrong with this braking system to make the front lock up with so many new parts already installed. He doesn't have the parts cannon budget to replace everything (again).

Just tryin to help a buddy in need here!

 

[Orpedcrow]

Do we know if “pops” let the master drain dry while swapping lines and such? Master could need a bench bleed.

If it were me (it has been several times) I’d start over from the top.

Bleed the master, then the prop valve, then each wheel again.

It sucks but a squishy pedal that won’t build pressure is either the master diaphragms bypassing or air in the system. Or an external leak (that should be easy to see lol)

 

[Orpedcrow]

Looking for advice on what else could be wrong with this braking system to make the front lock up with so many new parts already installed.

Just spit balling…

I’m not familiar with these older trucks, is that an ABS unit maybe?

Front and rear lines swapped?

Have you verified it’s only the front left locking? Or both?

 

[Orpedcrow]

Is it possible to install the prop valve backwards?

 

[RichLo]

Master is new'ish, it was in the truck for 5+ years before they started replacing downstream stuff.

The brake repairs started as a blown rear hard line so they replaced the bad lines. They replaced the proportioning valve because they said they couldn't bleed the rear. I've never had that problem personally with any kind of bleeding system but that could have just been clogged bleeders or a weak vac pump.

I have NOT verified that its one side or the other besides today after bleeding with the engine off, the drivers (problem) side released just fine after full leg pressure on the brake pedal.

 

[RichLo]

Vacuum booster pulling extra hard on them...????

 

[thinger2]

My Friend just dropped off his truck asking me to look into it quick since he is actively trying to sell it.

1989 K2500 350TBI ECLB Auto trans, 6-lug truck, vacuum brake booster.

OK, Lots of recent history with this truck and he is just trying to get the front brake to unlock and drive like normal so he can sell it. I'm just looking for quick checks to look into. I cant work on it until tomorrow night so asking for input tonight and tomorrow. If it was my truck I would go about the situation much differently but he needs cash quick so just trying to help him.

Recent history related to brakes: His Pop replaced rear brake lines from the master to the wheel cylinders and also the proportioning valve right next to the master, then tried to bleed the rear brake lines. He said the left rear couldn't bleed at all. He got the proportioning valve lock-open tool so he could bleed the rear, still didn't quite work apparently. I looked into the master and the rear reservoir is fresh and clean and the front is dirty.

He told me to not mess with the rear, its 'good enough' unfortunately. I'll still bleed that rear properly because its somebody else's safety. But that's just showing where his mind is at right now.

OK, history done, now onto the problem that he asked me to fix: This is a new problem since the rear brake lines and proportioning valve were replaced. (and fronts probably haven't been bleed). When you step on the brakes to shift from park to drive or reverse and try to go, the front drivers side is LOCKED SOLID. After he left, I tried to move it in front of my garage and it just spun the rear tires in either Drive or Reverse, non-directional. Then lightly tap the brake pedal dozen times and it drives like normal. Come to another stop like sitting at a stop light, same thing... tap it a dozen times and drives like normal.

My first step was going to be just bleed the whole system properly tomorrow but please any other ideas welcome!

The front rubber 'looks' ok but that would be my 2nd suggestion to him to replace. But ballooned rubber wouldn't let loose by tapping the brake repeatedly, right? Those just hold pressure and gets the rotor hot from my experience. These rotors weren't hot when he got here (30 min drive)

I've never replaced a proportioning valve or had to use a special tool to bleed it, could something be out of adjustment with it?

The left front was the worst but it could very well be both fronts locked up.

A bad hose that locks the caliper will usually release with a couple of taps on the brake pedal.
The inner lining of the hose where it terminates at the fitting rots and it kinda turns into octopus tentacles.
It blows open under pressure but sucks back under release and plugs the hose.
A couple of taps at stop can be enough to release the caliper untill the next hard pedal stop.
When those lines internally rot you get random chunks of rubber sticking and then coming loose.
You cant just bleed the rears and not touch the fronts.
It doesnt work that way.
There is no rears are good fronts are bad dont bleed the brakes situation.
Not since the early 1970s when the "split system" became federal law.
You can run front to rear isolated systems on forklifts and farm tractors and on heavy equipment.
Not on the road.
Whoever told you to not touch or bleed the rear brakes is way mis-informed.
Ill have to read back a little bit but is the same guy who replaced the rear brake lines the same guy who is telling you not to touch them?
Thats sounds a bit sketchy and skeevy to me.

 

[Schurkey]

In no particular order:

1. The vehicle does not have "a proportioning valve". That is, not a separate one, anyway. It has a Combination valve. The metering valve for the front brakes, the proportioning valve for the rear brakes, and the safety switch in between, all in one brass body that can't be installed backwards.

Aftermarket combination valves are known to leak air IN without leaking fluid OUT.

The metering valve section of the combination valve has separate outlets for the two front brakes. "I suppose" one of them could be occasionally plugged, leading to the one brake sticking on. However, as I said previously--I'd be looking at the caliper before I messed with anything else.

2. The thing with the bleeder screw on top is the early, iron-body Rear Wheel Anti-Lock device (RWAL).

3. He blew a rear brake tube. Therefore, the master cylinder went dry at least on the reservoir for the rear brakes. Bench-bleed the master, re-bleed everything afterwards. You might be able to get by with merely removing the master cylinder nuts from the booster, pull the master forward and tipped "down" a little in front (flexing the steel tubes--don't disconnect them) then "tickle" the primary piston with a wood dowel or Phillips screwdriver, to hopefully push any air bubbles back into the reservoir and out of the master cylinder itself.

 

[someotherguy]

Hoses replaced and brakes still locking up.. man, that sucks. Mind you I'm not telling you to go ahead and replace the master and booster but that's where I ended up, yet on a totally different truck. My wife's '06 Silverado SS, and I'm pretty sure my '06 SS will need the same parts soon, too. My first suspect in lockups like this is always the hoses. I replaced them on her truck and it would still lock up the fronts after some driving. I could mash the pedal a few times and they'd let go temporarily. I pulled the master cylinder away from the booster and discovered it had been leaking into the booster, and the booster was FULL of brake fluid. Replaced them both, and while using a screwdriver bench-bleed style to pump the fluid out of the old master so it wouldn't sit there and drip on my floor, the piston had a rough feel to it like it was dragging in the bore.

Don't fire the parts cannon yet; it gets expensive.. just some food for thought.

The hoses are always the #1 suspect if they're old. I replaced hers because they appeared original and had 134K miles on them. All 3 hoses, fluid, master cylinder, and booster I'm about $500 in that repair, but the brakes no longer lock up. (I went all new GM on the hoses, master, and booster. I don't trust remanufactured brake parts.)

Richard

 

[Erik the Awful]

Bench-bleed the master, re-bleed everything afterwards.

+1

I could regale you with the story of some racing teammates and I trying to bleed the brakes on our BMW at Sebring. We bought the race shop completely out of brake fluid, and went through almost all of the fluid before we realized the guy responsible for keeping the master cylinder filled hadn't removed the filter cup and the master cylinder kept running dry.