96 expedition brake sinking hissing

[indoorairpro]

I have a 96 expedition that only leaves the driveway when there is snow on the ground. I run it once a month to keep it lubricated. I decided to take it for a test run this month and had very little braking ability. Pedel sinks and hisses when depressed. Does not build up pressure when pumped.
I did some test. turned off and tried to depress pedal verylittle movement. Turned on car while maintaing pressure and pedal sinks. If I understand correctly this means brake booter is working.
Viewed mater cylender while son depressed brake a few times and did not see any movement of fluid on down stroke and saw some agitation of fluid on upstroke of pedal.
Sounds like air is escaping from under from part that travels through firewall when pedal is depressed. Is that normal. any other suggestions thanks in advance for your help

[wwest]

Originally Posted by indoorairpro

I have a 96 expedition that only leaves the driveway when there is snow on the ground. I run it once a month to keep it lubricated. I decided to take it for a test run this month and had very little braking ability. Pedel sinks and hisses when depressed. Does not build up pressure when pumped.
I did some test. turned off and tried to depress pedal verylittle movement. Turned on car while maintaing pressure and pedal sinks. If I understand correctly this means brake booter is working.
Viewed mater cylender while son depressed brake a few times and did not see any movement of fluid on down stroke and saw some agitation of fluid on upstroke of pedal.
Sounds like air is escaping from under from part that travels through firewall when pedal is depressed. Is that normal. any other suggestions thanks in advance for your help

"...sounds like air is escaping..."

More like being 'captured'....

The vacuum boost motor/diaphragm is vented to atmosphere inside the firewall. As the engine vacuum pulls the diaphragm forward the hiss you hear is atmospheric pressure filling the opposite side.

You may just need to drive enough to re-seat the pads.

[Cuda_Jim]

never mind ...............brain fart

[indoorairpro]

That makes sense. When you say drive it a while , I drove about 5 minutes probably hit the brakes about 10 - 15 times would you way that would be enough to seat pads? I was leaning toward a bad master cylender, thought maybe I blew something out the first time I hit it after being idle for 6 - 8 months. Trying to find out how to detirmine if master Cyl is bad. Would you happen to know ?