Auto Parts: 86 Ford F150 shutting off / starting up, band clamps, vacuum line

Question
Hey Mike,

 I got an 86 F150 4x4 with the 300, I installed a new master cylinder a couple of weeks ago, no problems.  Now the brakes are soft when the truck is on and running and if you hit them at a normal rate the truck just dies.  I did replace 1 vacuum line a few days ago because I thought it was the culprit.  Now when the truck shuts off it barely wants to start.  I had the starter and battery checked both turned out good.  I am not sure why the truck shuts off and doesn't want to start? Ideas?

Thanks,
Damon

Answer
    So it cranks well but won't start and doesn't run good when it does, right?  That's almost certainly a vacuum leak.  I'd be willing to bet that you need a new booster.  In fact, you probably never needed a new master cylinder.  What happens is the seal inside the booster, between the vacuum chamber and the back of the master cylinder, goes soft and starts to leak vacuum.  The vacuum begins to suck fluid out of the back of the master cylinder, and the fluid runs down into the bottom of the booster, ruining it.  You can sometimes see the brake fluid stain at the bottom of the booster where the band clamps the front and rear halves together.  If you bought a rebuilt master cylinder, try to get them to take it back as a defect and buy a rebuilt master cylinder/booster combination.  If they don't want to do that, or if you bought a brand new master cylinder (always a good idea, IMHO), then keep the master cylinder and buy a rebuilt booster.  Make sure that you get a new check valve with it and make sure that the vacuum line from the check valve to the manifold is replaced (it's pretty old by now; don't cheap out here, get a good sturdy hose that won't collapse).  Bleed the master cylinder again, at all four wheels, just in case some air snuck into the system.  And bleed it the right way, with a clear hose over the bleeder.  Submerge the end of the hose in brake fluid so that bubbles can't go back up the tube and watch the hose until you don't see any more bubbles.  Oh, and whoever you've got pushing the pedal for you, don't let them stroke the pedal all the way to the floor; stop with a little space left at the end.  When they machine the bore of the master cylinder, they can't go all the way to the end.  So they leave the bore unfinished at the very bottom and it can damage the seals on the front piston.