on my 87 e350, i've been noticing a hard brake pedal sometimes for the last several days, so tonight i hooked up a vacuum gage to the system to see what was up. vacuum would reach a maximum of 21" but took 25 seconds after starting to reach it. pressing the brake pedal would drop the reading by anywhere from 3-5", and quickly releasing the brake could drop it by as much as 6". with the extremely slow recovery of the vacuum pressure, this can easily result in almost no vacuum pressure, which is obviously causing my hard pedal.

so all that said, does it sound like my brake booster is leaking and demanding more vacuum than the pump should be able to provide, or is the pump failing and thats my problem?

if the vacuum pump is the problem, it'll be the 4th one in 2 years, but they've always failed quickly, not slowly like this

i took a video of vacuum gage readings while playing with the brake pedal, i'll post it in awhile if i can get it uploaded somewhere tonight (yes i know how, just connection problems)

 

Wait a half hour after shutting down the truck and pull the vac line off the booster-- if it makes a strong sucking/whoosh sound I'd say its the pump, if not you have a leak somewhere either in the booster diaphram or one of the vac lines.

Hope that helps diagnose it,
Joshua

P.S. You should be able to pull that line and have vacuum even the next morning. So it's not like a short window that it holds vacuum.

 

X2 with the above!

 

the vacuum gage i was playing with dropped to zero after several seconds of turning the engine off, but am i reading this right that you're saying to pull the check valve out of the booster to see if it still holds vacuum... i'll give that a shot after i drive to where i'm working today. thanks for the idea

 

i finally got the video of the vacuum gage and brake pedal relationship, lets see if this works
VID_20130513_224835.mp4 Video by redneck_from_index | Photobucket

 

well, the heck with the video there, i got a real diagnosis tonight, and i have it solved.

the real problem was the vacuum connections at the VRV, that rubber fitting thingy was loose enough against the VRV that it never sealed, and pinching off the hose from the vacuum tree to it would increase my max reading from 21 to 23.5. and with it how it was, i would stop the engine and watch the vacuum gage quickly fall, reaching zero in about 5 seconds, but the brake booster still had vacuum in it, meaning it wasn't the problem.
so to solve this, i first replaced all the hose in that circuit, with no change in performance, and then found that rubber fitting was loose, and for lack of a better option within reach, i cut 2 pieces of 1/4" heat shring tubing about half an inch long, slid them onto the VRV fittings, shrunk them, then slid the rubber fitting back on, now a much better fit. now i had no loss of vacuum at all in a 5 minute test with the gage on and engine off.

but during all this testing i found ATF in the hoses, which means i get to replace the modulator on the tranny sometime soon.