I had a broken brake line and it drained the master cylinder dry. Replaced the brake line, filled the MC and bled the brakes. Plenty of fluid at each calliper. With the truck off there is plenty of pedal, with the truck running the pedal goes to the floor and very little stopping power. I'm thinking the MC may be damaged . Any thoughts?

 

Only difference running vrs not running is the vacuum booster, what all or else did you do to it?

 

nothing

 

When you bled the brakes was the engine running?

 

It still sounds like a bleeding problem. Did you start with passenger side rear first, then drivers side rear, then passenger front, then driver front?

 

did you bench bleed MC? you most likely have air in it. you can also bleed MC on the truck. they sell kits for MC bleeding, few bucks well worth it.
thereafter, you'll have to rebleed brakes. if yours has ABS, when done with regular bleeding, drive slowly and slam bad on brakes a few times after acceleration. air may be trapped in ABS module cylinders, and that's how you get it out.

 

well, yes, but he didn't replace the MC. You can eventually get all the air out of the system, but without bench bleeding the MC you are pushing air into the system. Most new or reconditioned MC come with a "bleeder set up" that comprises of a plug valve and short length of hose attached on each resevior.

 

he drained MC dry. that's like as if installing new MC which ever way you look at it from hydraulics perspective. Air went in, and who knows where else. Might as well be combo of MC and ABS unit trapping air.

 

I agree.

 

bled the brakes with the truck running, took care of the problem, thanks for all the help!