chevy 2001 2500 chevy HD. I had to change a leaking master cylinder
I purged the hydro side by cutting the wheel from stop to stop with the motor off. Then purged brakes as per chevy( at least 10 times). I got little pressure and when I start the truck the pedel goes to the floor.
What am not doing?

 

I have alsway had the engine running when purging the brakes. Start with the rear right then rear left, front right then left. Make sure the reservoir stays nearly full.

 

did you bench bleed or bleed the master cylinder itself? Normally you shouldn't even need to bleed the wheels if all you replaced was the master and it was bench bled.

 

The new (used) cylinder came from a junk yard I did not think I had to bench bleed. Is there anything I can do now that its installed?

 

Bleed the brakes with the engine running.

 

you can do it at the wheels but it'll take a while and you probably won't get all the air out without using a scanner to do the automated abs bleed

I'd try just loosening the lines at the master and bleed the master and see how it ends up. You'll probably have to do a full bleed now since theres probably already air in the system. The only way you wouldn't have needed to bench bleed is if it was full of fluid when you put it on and never went low/empty.

 

Don't bleed with truck running booster will not help bleep whole system
Get a water bottle get all water out get a little hose that fits over bleed pin a bout 2 ft fill bottle up over half then put hose in bottle then step on break as much as you can each wheel this way will let you do it alone and no air going back in system
Make sure you do far to near to master

 

Each to their own. The whole process is much easier with the engine running. Try it each way and decide for yourself.

 

I am Doug
Anyway with truck running there always a chance of moving and with no brakes not cool

 

I bleed at the master and get some good pedal with truck not running. When truck is running no pedal at all. I tried purging all lines again, and again and again With truck off i get pedal start truck no pedal
please help. Do I give up and tow to a garage?
Thanks