Things went mushy on me last night. tooled around a bit today and the pedal goes all the way to the floor and makes a hissing sound like air discharging as it goes. near the bottom of the travel the brakes begin to work albeit at a highly reduced rate, then if i really stand on it, i stop. If i pump the pedal a bit it seems to build up some pressure then they function normally as my pedal sinks slowly to the floor as the pressure falls off. normally i would go straight for the master cylinder as the fluid is not draining out, but i have never had a car with a booster either at least i dont think i have. in fact not even sure what a brake booster is? thoughts on where i should start?

 

Master cylinder going bad.

The usual test is to pump up the brakes, press your foot on the brake pedal and hold it, if it goes to the floor it is usually bad. (engine off)
If you can pump them back up but it fades again then it is the master cylinder.

The hissing sound you hear is the vacuum leaking out. The brake booster uses engine vacuum to multiply the pressure you put on the brake pedal.
If the booster goes bad you can pump up the brakes by pumping the pedal repeatedly.
If pumping up the brakes does not work then the master cylinder is bad.

Start the engine, have your wife pump up the brakes while you look/feel/listen for the hiss, it will most likely be coming from the hose that goes from the MC to the engine.
Check for cracks, replace that hose if needed.
If that is all good then replace the MC.

 

awesome thanks... I was poking around in that area yesterday maybe i knocked something loose

 

im not finding a HOSE per say it all seems to be steel ilnes going from the master cylinder. Unless I am missing something it was a bit dark when i looked.

 

The hose runs from the BOOSTER to the engine (maybe 1/2" dia.)

The hose may be off, cracked or connection fitting cracked, you may also have a bad diaphram ibnside the booster.

I'd use a piece of hose (1/4")? from your ear to all around that area, you'll locate the leak.


luck,greg

 

ok so i can pump it up when the motor is off, but as soon as i start it the pressure releases on the pedal. also, strange thing after sitting for a while the resorvoir was really low, i was not leaking before and still cant find any sort of leak now. when it was empty obviously it was not stopping at all.... but when i filled back up it went back to being able to stop if i pump the pedal slowly and let the pressure build up. this is the current status. i will check for that hose tomorrow, and try to locate the source of the pressure release.

 

With the vehicle off put your foot on the brake until you feel resistance. Start the vehicle with your foot on the pedal. It should e pulled to the floor. If there is little or no effect, your booster is bad.

 

Hello all, I thought it would be good to continue building on this thread.

I am trying to determine weather I have a bad master cylinder or brake booster....I push on the pedal while the Rover is running and I hear a hissing sound. I have replaced the pads with Akonobo pads. The system was bled properly about a year ago.
So how can I determine if I have a bad booster or master cylinder or both?

 

Trying to restart this thread since I have the same question. Thanks

 

If your truck has the same symptoms, the answers haven't changed in the last nine years.