brake pedal goes soft when engine is running
 

I'm certain there's no air in my brake lines as I've bled, pressure-bled, reverse pressure bled, and bled again. The pedal feels firm when the engine is off (I think it could be a bit firmer but there's good resistance from the pedal). When the engine is running the pedal drops to the floor with the slightest press and I have to cram it down to stop the rear tires when idling in reverse (or drive). Rear tires are off the garage floor. My cylinders are new, lines are new, rubber hoses are about 1.5 years old but have only been driven with once.

Howdy:
It's been some years since I've experienced this, but as I recall it, a rebuilt Master Cylinder (and rebleed of the entire system after installation), provided the cure. Just my $.02, hope it helps.
cowboy33713

Sounds like your booster is bad. Do you have the vaccuum line hooked up to the booster?

yes, vaccuum to the booster is hooked up. How could a bad booster cause the problem? If it wasn't holding vaccum wouldn't that make the pedal firmer instead of softer? Can a booster over-boost?

yeah, you are right, I was thinking about something else. I was thinking bad diaphram, but it would give you a firmer, not softer pedal

Well, new master cylinder and re-bled, same problem. I made sure the rear brakes are adjusted nice and tight - even made them a hair too tight to be sure. Still have no resistance on the pedal with the engine running.

How much resistance should I feel in the pedal with the engine OFF? Seems firm but to me it seems like it could be firmer. Regardless, a little bit of pressure on the pedal does engage the brake to the point that I can't turn the tire by hand. Granted the 350 might be a bit stronger...

How much cam are you running? You might have too much cam to make enough vacuum for power brakes at idle.
Did you check the rubber brake lines? They could be bulging when you hit the brakes.
Other than that, I have no clue, so I'm spent.
Todd

if I disconnect the booster, fire up the engine, put it in reverse with the wheels off the floor, it is still a pita to stop the tires. Takes more pedal pressure than it seems like it should. So I'm going to do some more bleeding (why not) and inspect the flexi lines to see if something's killing my fuild pressure.

Does it WHOOSH Inside?
 

Do you here a Whooshing sound inside when you hit the brakes? If so, bad booster seal. If you have rebuilt the engine since you last used the brakes, then toddtheodd could be right if you replaced the cam with one with ALOT more duration. In that case, you may need a supplemental booster can. I have one from Cam Dynamics on my Chevelle. Nobody thinks that boosters go bad, but they DO. I wish you luck, Hot Rod

If air rushes from the booster hose after you shut the truck off off its ok. Go out and pull the hose off of the booster after you shut the truck off. If its ok its the master. IMO Could be the vacum issue also.

Howdy,again.
Did your truck come with power brakes, or is it a conversion? Is there an adjustment to the rod from the pedal to the master cylinder , for pedal height before the pedal is depressed( rod too short)? ( haven't gone out to check mine; everything I've ever changed out was a direct remove/replace). Does anyone know if a malfunctioning proportioning valve might cause this?
-cowboy33713