Join Date: Nov 1999
Location: Central California, USA
Re: RPM increase when brakes applied?
When he depresses the pedal, the engine should speed up a little then settle right down. If it continues to idle rough, there is a vaccum leak in the booster. Either from the pushrod side or the master cyl side.
The rear of the master cyl has a horizontal hole on the rear just below where it touches the booser, plug this and test it by applying the brakes. If you have a leak in the booster front seal, it will suck through this hole.
Excessive pedal travel due to air in the lines, can cause the engine to run rough like this but it should even out fairly quickly, not keep running rough.
Around the pushrod connected to the pedal, there is a foam "filter" it helps quiet the air rushing into the booster. this foam can rot and get sucked into the booster and perhaps into the reacton valve inside. This would cause faulty valve operation but not loss of vaccum.
When you pull the distributor hose, it's retarding the spark so the engine slows down as it should.