If you're referring to an engine-driven vacuum pump, those are only used on some diesel engines. 20" hg is more than enough to run a vacuum booster. If there wasn't enough vacuum, the pedal would push very hard and it would not feel mushy or go to the floor.
Internal leakage in the master cylinder can cause a mushy pedal too, but you usually won't be able to pump it up. A clue is that the pedal might hold while you hold steady pressure on it, but it will suddenly start to fall away when you ease up a bit and vary the amount of pressure.
When the low pedal is related to rear shoe adjustment, you'll get a nice hard pedal after a few pumps and it will stay up as long as you hold it, then after releasing it and immediately taking another stroke, it will sink too low again. It takes some time for the rear shoes to retract. When you pump the pedal you're taking a new bite of fluid and pressing the pedal before the shoes have fully retracted. Eventually you have enough fluid in the system to push the shoes out far enough to contact the drums. That's when the pedal feels firm and high. When you release the pedal for more than a few seconds, the shoes have time to fully retract. With the next pedal stroke the shoes have to again move so far that the pedal goes down real far.
Wednesday, February 23rd, 2011 AT 6:53 PM