break pedal goes to floor when engine is running

[Divh]

i have a 1989 ford ranger when engine is off i have a good pedal there is no air in the line when truck is running pedal goes to floor and rpm goes up and down thinking it mite be the break booster cuz when i hit the pedal it also makes a wisshhh sound any help on this matter would be very helpful

 

[RonD]

Do the brakes work when engine is off?

i.e. Does pushing down on the pedal actually engage the brakes?

Yes reads like power booster has a leak between vacuum side and cab side, so that will need to be replaced, just not sure on the master cylinder.

On the engine side of power booster the vacuum hose to the running engine creates negative air pressure in 1/2 the power booster
In the cab where the brake pedal push rod goes into the power booster is an atmosphere valve, when you press on the pedal that valve is opened and the 15psi(at sea level) air pressure rushes in to "assist" you in pressing down the brake pedal because of the negative pressure in 1/2 the booster.

The engine RPMs changing and the wooshing noise when pedal is pushed means the booster diaphragm has a leak.
But you should still have pedal pressure unless booster push rod is no longer connected to master.
If brakes don't work with engine off then that is what has happened

 

[Divh]

They engage when engine is off but think I mite have solved my problem finally remembered there is 2 different break boosters for abs and none abs in my hurry doing new break lines i completely bypassed abs unit so going to hook that back up and see if that solves my problem will do that tomorrow after work and will post up if it's fixed

 

[Divh]

well its break booster picking up new one tomorrow