Brake Booster vacuum leak

[Anonymous]

On my '85 F150 with the 300ci and auto trans., I recently replaced the master cylinder. Brakes just went out without warning over an afternoon. After replacing the master cylinder and bleeding the brakes, I started hearing a hiss under the dash every time I stepped on the brake pedal. Wasn't there before. It's getting worse and now hisses constantly unless the engine need for vacuum exceeds the leak, then it goes away and comes back when I reduce RPM's. It appears to come from where the plunger off of the pedal goes through the firewall. Before I spend the $65 and labor to replace the brake booster, could it be anything else? Maybe the booster check valve? I also read, in a manual, about some kind of vacuum safety switch under the dash, in the brake pedal area?

 

[wsa111]

First check the clearance between the rod from the master cylinder & the booster ,make sure the clearance is in specs.

If the rod is within specs, sorry replace the booster, there is a vacuum leak in the diaphram. William

 

[Anonymous]

Yep, that was it. Replaced the booster and the hiss went away. Easy job, took less than an hour. Idle is much better now too. Thanks guys!