Brake pedal sinks after prolonged braking
 

My brake pedal has been pretty soft lately, I originally thought it was the master cylinder because it would slowly sink when pressure was applied. I replaced the MC with a good used one and bled the system, but the problem is the same. Would I be crazy to suspect that one or some of the hoses are old enough to be swelling with the brake pressure?

If the pedal slowly continues to sink while you're holding it, it is because fluid is escaping the pressurized part of the system at some point. If it's not a master cylinder seal that it leaking, you will lose fluid. The hoses will pressurize immediately, and thus won't cause a sinking pedal. They may cause increased pedal travel, in which get them off your car immediately (but I doubt that this is your problem).

You are probably loosing fluid somewhere, chack level. If not at one of the hoses, good chance the booster is catching the run off. Did you replace the o ring between the MC and booster?

No, I just used the old o-ring. It's possible that the MC could be leaking into the booster? I'll get a new o-ring and see if that helps

Check the o-ring first. Make sure the area is clean. I had the same thing and it was 1 slightly leaking caliper. It wasn't leaking enough to see fluid because dirt was gathering on whatever fluid was coming out. Look for maybe a dirt clumpy caliper.

Yes, the master cylinder can leak into the brake booster. It can fill up with fluid!

Check all the hoses for signs of blisters in the outer covering. If any are present, replace all four. In fact, if they don't look new, replace all four, they aren't expensive.

Flush the fluid completely while you are at it, too -- unless it looks new, it needs to be changed (every two years!).

If you have no leaks and are not loosing fluid, your used MC is no better than the old original one.

Peter