Stratton_Silverado1500

Thanks Richard! It took me about an hour just to get the top right bolt out of the firewall the first time. And the clip on the brake pedal can be a pain to get back on. But I can swap the whole thing in 20 minutes now. Partially why I balked at 3 hour labor charge for a booster change at the dealership. The Cardone replacements did all come from the same shop. All things considered there's a good chance you're right about a bad batch. I appreciate the help along the way!

Update: I checked a couple of other vehicles and couldn't find a spacer between the booster and firewall, and couldn't find evidence of this part online. It's still possible I threw away a spacer with the original booster core. But without a good path forward other than putting on yet another booster—a fairly reliable part—or a third new master cylinder, I did what worked the first time and gave the pushrod a few turns to shorten it. She's driving fine and I've made it from Charleston to Kentucky so far en route to Utah. If there are further developments or knowledge gained, I'll update this thread. It wasn't the start-to-finish SOLVED post I'd hoped for, but oh well. Even the dealership just wanted to throw another booster at it. Sometime in the future I may pop a GM booster and master cylinder combo in to see if that does it.

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.

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

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.

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!

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.