You should consider starting a new question for a couple of reasons. First, this became a private conversation between just two people. As such, none of the other experts will get notifications a reply was posted, so they won't have the opportunity to respond. That may not get you the help you need.
Second, these get categorized by model and by type of problem, to make it easier for others to find when researching the fix for something similar. The solution specific to your model won't show up in those lists.
If you have a different model or year, some of our suggestions could be wrong or might not apply to what is in this question. We could inadvertently send you down the wrong diagnostic path. I'm going to try to help you here, but if we need to get the other experts involved, go here:
https://www.2carpros.com/questions/new
to start a new question specific to your model. Please be sure to list the engine size and mileage. Diagrams and systems are often very different with different engine sizes, and we look at the mileage when making generalization as to best suspects.
If there was no problem related to locking brakes before the power booster was replaced, there's two things to look at. The first is the length of the pushrod coming out of the front of the booster. Those are usually not adjustable on U.S. domestic car models, but they usually are on imports. That doesn't mean those are maintenance adjustments or that they're meant to be adjusted. Those are set during the assembly process and shouldn't need to be changed later. If one is not set correctly, it will hold the master cylinder partly applied. That will prevent heating and expanding brake fluid from freely returning to the reservoir. The trapped brake fluid will expand and apply the brakes harder and harder.
The next thing to consider is the adjustment of the brake light switch. If that got bumped or reinstalled out of adjustment, it can also hold the brake pedal down a little with the same resulting locked brakes. A quick way to verify one of these is the cause is to loosen the mounting nuts for the master cylinder and let it slide forward 1/8". That is enough to let the brakes release.
If this locking problem did not occur until a week or two after the booster was replaced, a possible cause is the brake fluid is contaminated with a petroleum product such as engine oil, power steering fluid, transmission fluid, or penetrating oil. Those cause the rubber parts, including the seals in the master cylinder, to swell and grow past the fluid return ports, blocking them. That takes about a week to show up and is a very expensive and time-consuming repair. We can discuss that further if it becomes necessary.
Check those things I mentioned, then tell me what you found.
Sunday, July 9th, 2023 AT 5:35 PM
