The same thing happened to me. I have one of those all in one upgrade
kits that comes with the booster, master cylinder and proportioning
valve and bolts onto the firewall of my 1964 GMC. Through a lot of
sweat, considerable hydraulic press work, and really painful monday
mornings I have 4 wheel disc brakes.

I recently decided I had an air leak, fixed it and bled the brakes
again. Had firm pedal, everything perfect. Started the truck to take a
spin around the block and the PEDAL DROPPED TO THE FLOOR!!! As soon as
I cut the engine the pedal came right back. The brakes were locked
pretty good with the engine running and I had to give it some gas just
to get it back into the driveway.

I spent 2 hours in the dark adjusting the length of the pushrod,
convinced that it was slightly too long or too short and was causing
the booster to prematurely evacuate or something.

Eventually I called up Brothers Trucks with my woes and they warrantied
the booster for me, their supplier had one in stock and traded it out
the next day (I didn't have the booster at the time). Magically
everything was fixed great, beautiful.

alan

From:        Jonathan Ward - view profile
Date:        Sat, Jan 26 2002 5:13 pm

In almost 35 years of wrenching, I've never experienced anything like
this.  My manual (Haynes) is basically useless, and I need some help.

Working on a 1978 Ford F350 flatbed, 10,000 pound GVWR.  400 CID
engine.  Strange behavior from the power brakes.  When the engine is
running, the booster is applying brake pressure without any pedal
actuation.  Turn the engine on, the brake pedal drops a bit, and the
brake lights come on.  Turn the engine off, the pedal returns to its
top position.

When I'm head-down in the cab messing with the brake pedal by hand, I
hear air rushing (vacuum?).  The sound stops when I push the brake
pedal a little bit, then returns when I release it..

When I pull the check valve out of the booster, nothing changes.
Strong vacuum (audible) at the check valve.  Plug the end of the check
valve with my thumb, and the engine stumbles and dies.  Seems like
that shouldn't happen.  (If it makes any difference, all vacuum hoses
are new, carburetor rebuilt, everything torqued to specs.)

Is this a brake booster check valve, or is the booster toast?  It's
definitely adding power assist, but all the time.  The "money is no
object" method would be to just replace everything on the theory that
it's probably 24 years old and needs replacement anyway.  However, the
booster runs about $160, so I'd like to avoid replacing it if it's not
necessary.

Your thoughts?  Thanks in advance!