wow . . i thought of doing mech engineering as well. no time or money though. plus got a 2 yrd old. hey, are you at that UTI school?

 

USF tampa.

 

ok, back to topic. please ponder my rambling reasoning here and tell me if you concur or something else:

my ford 1980 3-inch-thick shop manual says NOTHING about troubleshooting the brake booster... only how to replace it. they seem to think that complex and detailed diagrams about how the booster works and it's tiny little components are more important than a 3 line paragraph about how to tell if the damn thing is malfunctioning. (way to go ford!)

so while i'm saving up for the booster, i did some detective work.

the vacuum line to it looks fine, (only 1 right?) not cracked or falling apart or anything. it's connection to the booster seems tight.
funny thing is that while the truck is running, the hissing is loud inside the cab coming from under the dash where the booster is but the hissing is hardly audible from the engine bay.

that makes me think that if it was the vacuum line leaking, i'd hear it from the engine bay MORE than i would from the cab. right?

also, once i shut the truck down the hissing slowly fades away and stops altogether after about 30 seconds. This leads me to believe that the vacuum line is working (no sound from engine bay) and creating enough vacuum to semi-run the booster... however, the booster is leaking somewhere on its rear-side and when steady vacuum is no longer supplied to it from the engine, it slowly bleeds off until it no longer has any vacuum.

what do you guys think? I'm not trying to beat this dead horse, but just want to make sure I'm not replacing a part if I don't need to.
Thanks much.

 

replace the booster.

 

thanks all for the input.

brakes are MUCH better now. i never thought the pedal would feel so different!

hissing is gone and the booster must have been creating a small vacuum leak because after replacing it (and the hose) the idle smoothed out.

thank again guys.

 

Glad to hear you got it. I have gotten into the habit when I get a "new" truck of giving it a tuneup and fixing vac leaks before doing much else... they always run a little better or eliminate complications when trying to resolve other issues.

 

lol datch, just so you can sleep better at night, the reason you were hearing the hiss is because the boosters diaphram was leaking . . .inside the booster. the reason no manual will show you how to fix it is because basically, unless you're God, no one will be able to get the booster closed and sealed properly again lol. its good that you replaced it anyway cause thats what you needed to do. the booster is a "non-repairable" part. its only replaceable. fyi

 

I'm so glad I randomly found this thread. Brakes weren't working really hot, driveable, but giving me a heck of a leg workout. I figured I had some terribly fun problem with the brakes. After reading this I realized that that hisssssssssssss wasn't normal (I'm very new to these trucks, actually, to Fords in general). Went to a local salvage yard, picked up a brake booster for 11$ off an '86, replaced mine with the newer one.

Brakes work much better now, not to mention the cab being quieter. (So i can hear the exhaust leaks i need to take care of more prominently, hehe)

Thanks guys, the info on this forum is very helpful for those of us new to these trucks.