When doing a major brake job, such as replacing lines, repairing a caliper, and so on, it’s normal procedure to bleed the brakes in order to get the air out of the system and re-pressurize it. Sometimes this doesn’t work, though, and the do-it-yourself mechanic is left frustrated. Why won’t the brakes re-pressurize?
We often then turn to the master cylinder, assuming that it’s failed or isn’t pressurizing the system properly. That’s partially correct, but most often the problem is that the brake master cylinder hasn’t been bled when the rest of the lines were. A master cylinder with air in it will keep the brakes feeling spongy, whereas a failed power brake booster usually “locks up” the brake pedal, not allowing a press easily.