I've got a 99 Blazer, 4 wheel disc brakes. Driving home today the pedal started going all the way to the floor, and the "brake" light came on. When I turn the car off, I can pump the pedal and it will be firm, but as soon as I turn it back on, the brake light is on and the pedal will go all the way to the floor again. Pumping it while it is running does nothing, it does not get firm. The fluid level is staying the same.

Does this sound like a master cylinder, brake booster or air in the lines? Maybe something else?

Thanks for any help...

 

Sure sounds like air in the lines to me, but what would cause it to just all of the sudden get air in the lines?? I would start by doing a full inspection of the system to make sure that you do not have a leak somewhere...

Maybe do a full brake bleed and see what problems you notice from there. Start at the passenger rear, then driver rear, passenger front, and driver front last (order from furthest to closest from the master cylinder). Bleed until you get fresh, clean fluid out of each line.

 

Thanks for the advice...I looked for leaks and have not seen any, and the fluid level is staying constant. I'll give the complete bleeda shot, I'm crossing my fingers it is that simple.

I did replace a caliper 4 weeks ago, and bled only that one (did not do all 4). It has not given me any trouble at all until today, so I am not sure if that has anything to do with it or not.

 

Sounds like the M/Cyl to me. If you have not had to add fluid then you have no leaks, I.E. bad calipers, hoses, or lines. It is definately not your booster. That is only a "power assist" . If your pedal goes to the floor and then you can pump up the pedal but it won't stay up, then the m/cyl is bypassing itself internally loosing the pressure. For your replacement, get a new and not reman or used. The few bucks you spend extra for the new is cheaper than having to fix someone elses car because you couldn't stop in time.