Front Brake Locking Goblin

[Stratton_Silverado1500]

First time poster. I've read a lot of forums about this problem over the last month, so I'm posting my story—and will post the solution—in hopes it can help someone else. I'm not sure I've seen this exact scenario across the dozens of posts I've read. 

This is a 2007 Silverado 1500 Classic 4wd Z71 5.3

My front brakes keep locking up. Disc in front, drum in back. I've bounced this across my pro mechanic friends already, and after 20 years of shade tree wrenching, it's now at the dealership, because even the GM specialist helping me this week couldn't figure it out. 

Step 1: It started with a front end rebuild (new upper and lower control arms, BJs, pitman arm, idler arm, stab links, front and rear shocks, and a new passenger side axle seal). Calipers were stuck when I broke it down so I replaced them too. Bleeding went as it should with the ignition off. With the truck on, pedal went to the floor and wouldn't firm up as I bled. I popped the master cylinder off and saw that the booster was full of fluid. I don't know if it was oil or brake fluid—I took it out and swapped the core for a new one (AC Delco). 

 

Step 2: Test drive with new booster and new calipers resulted in locked, smoking front brakes. Charge up the parts cannon—new master cylinder time (Autozone). Still smoking on test drive.

Step 3: I then discovered that my helper had put the calipers on the wrong wheels, upside down. I corrected that and also put on new flex hoses up front. Still smoking, and now I'm seeing brown flaky gunk in the brake fluid of the new master cylinder reservoir.

Step 4: I put on a second new master cylinder (Autozone again), got a pressure bleeder, and bled until it ran clear from all four wheels. (No more gunk in master cylinder since then). Rear driver side bled a bit slower than the others, but still bled until it was new fluid. Test drive - still smoking. Every time, it starts out fine, then builds pressure until brakes are locked and the pedal is tight. I discover I can unlock the brakes to drive it home by cracking a bleeder screw on a front caliper. 

Step 5: Finally took it to my mechanic buddy. He suggested the push rod in the booster might be too long, forcing the brakes to always have pressure. These aren't supposed to be adjusted in GMs, right? But we popped it out, shortened it four full rotations, and snapped it back in. Truck ran fine for a week but pedal was squishy. 

Step 6: Decided to pop pushrod back out and bring it out one and a half turns to fix squishiness. Otherwise it had been running great. After lengthening push rod, brakes lock and start smoking again 10 minutes into test drive. So I replace the brake booster with a brand new one, factory length push rod. 

Step 7: Brakes are no longer smoking, but they're definitely locking up intermittently and getting way too hot. I jack up the front and the wheels are locked up. I pull the vacuum hose off of the booster and the wheels immediately free up. Mechanic buddy is at a loss and recommends I take it to the stealership, where it sits now. 

The pads were new with the new calipers, but I never changed the rotors. With both locking up, it seemed upstream—especially since I can release them by cracking bleeders or pulling the vacuum hose off the booster. 

Would love your thoughts if you have ideas. At this point, mostly posting to start the thread and post the solution when I have it.  
 

Edited January 19, 2022 by Stratton_Silverado1500

[richard wysong]

you were on the right track shortening the pushrod, its definetly the master cylinder not being able to fully release.the squishyness was having the calipers upside down. when you saw fluid in the booster did you change the booster or the master?too late now that its at the dealer but if you adjust the pushrod again I think it would cure it

[Stratton_Silverado1500]

Richard, I changed the booster first when I saw the fluid, and then changed the master after I put the new booster in and it was locking up. But since then, there's been yet another master and booster installed (so two of each since the issue started). 

When you pull the pushrod out to adjust it, it's almost impossible not to mangle the bracket that holds it when putting it back in. And that made me worry that it could fall out at some point. I know on many Japanese makes that the pushrod is meant to be adjusted on a new booster - there's a tool to measure it - but is that also the case with the GM ones? I'm tempted to go pick up the truck, adjust the pushrod again, and hit the road. But I've got small kids so I can't take chances with brakes, if that is indeed taking a chance. That bracket doesn't seem to want to come out of there, and with loctite on the rod threads, it's impossible to reach in and adjust it in the booster. 

Thanks! 

[richard wysong]

is it possible there was some sort of spacer attached to the original booster that was returned as a core? thats about the only thing I an think of that could cause this problem. it's probably too late to go to the store and look so you may end up in a wrecking yard to check

[richard wysong]

you may be able to pull up a parts diagram at GMparts.com or the parts dept at the dealer. thats the only thing that makes any sense to me. the fluid was probably from the master as there is no fluid in a vacuum booster

[KARNUT]

I had that problem on my 92. New calibers, shoes same thing. Finally I changed the rubber brake hoses problem solved. 

[richard wysong]

he has already replaced the hoses so thats not it. the problem with the hoses is rust buildup in the bracket that holds it to the knuckle pinches off the hose

[KARNUT]

10 minutes ago, richard wysong said:

he has already replaced the hoses so thats not it. the problem with the hoses is rust buildup in the bracket that holds it to the knuckle pinches off the hose

Mine collapsed inside. Looked fine outside.

[Stratton_Silverado1500]

When I mentioned I didn't know if the fluid in the booster was oil or brake fluid, that was based on a mechanic mentioning that too much vacuum can pull oil from the engine in. And given that pulling the vacuum releases my brakes, that seemed possible. Either way, there shouldn't be fluid in the booster.

Still waiting for dealership to call, but if they're not helpful, I'm brainstorming what to use for an airtight spacer between the booster and cylinder. I don't want to pull the pushrod out of the new booster and mess up the bracket again.

Looks like there's a spacer made for Ford boosters but not for GM.  

Edited January 19, 2022 by Stratton_Silverado1500

[newdude]

Being you have an 07 Classic, did Autozone get you the right booster and cylinder?  Did they perhaps look it up as an 07 new body?

 

GM #s you should have are 19419041 master cylinder 2nd design and 19209324 booster (also 2nd design). 

 

In Duralast, I see NM4109 or M4109 master cylinder and 54-74829 booster for 07 classic.

 

 

[Stratton_Silverado1500]

36 minutes ago, newdude said:

Being you have an 07 Classic, did Autozone get you the right booster and cylinder?  Did they perhaps look it up as an 07 new body?

 

GM #s you should have are 19419041 master cylinder 2nd design and 19209324 booster (also 2nd design). 

 

In Duralast, I see NM4109 or M4109 master cylinder and 54-74829 booster for 07 classic.

 

 

Both master cylinder replacements I put on were the NM4109. Thanks for pulling that info though! 

 

[Stratton_Silverado1500]

Dealership wants to replace the Cardone 54-74829 booster I’ve put in twice with a GM 19418518. $350 for the part, $575 for labor. 
 

I’m not optimistic that changing a part I’ve changed twice will do it. Trying to decide if I should prepay for the GM booster to get it tomorrow morning and put it in myself. 
 

Leaning heavily to Jerry rig push rod shortening trick and hitting the road to Utah tomorrow. Ugh. 

[asilverblazer]

Unplug the booster and plug the vacuum line, at least confirm whether the booster is the problem or the master cylinder.

[richard wysong]

the spacer would be between the booster and the firewall, if they don't show 1 I bet its not the correct booster. think about it, the problem started when you changed the booster when you should have changed the master. there is NO way to suck oil from the engine to the booster or to make TOO MUCH vacuum. any fluid in that area has to be brake fluid from the master cylinder seal so that was bad advice from your friend. the only reason to replace a booster is a vacuum leak or lack of boost. all it consists of inside is a diaphram and a spring with a check valve on the end of the hose . check and see if the spacers missing and if not go with the gm part

[richard wysong]

or go to the boneyard and get a used 1, the things never go bad and if there is a spacer you can get it and save some $. in 50 years as a mechanic I have only replaced a couple for vacuum leaks