Bench-bleeding is when you mount the new master cylinder in a vise, fill it with new fluid, then pump it manually to expel the air. It's a standard part of replacing any master cylinder. If you don't do that, on some systems you'll never get all the air out when the master cylinder sits in a tilted position. On those where all the air will bleed out, it is going to have to work its way down to all of the wheels to be bled out. There are tricks mechanics use that eliminate the need for bleeding at any of the wheels when they replace the master cylinder, but they still have to bench-bleed it.
A low pedal can also be caused by rear shoes out-of-adjustment. The clue is the pedal will become firm and higher after stroking the pedal rapidly a few times, then holding it. Once it is released, it will be low and mushy again the next time it's pressed.
Wednesday, May 11th, 2011 AT 9:39 PM