69 ChevyC20 brake conversion problem

[Merbob]

I have recently converted my C20 to front disc brakes. I purchased the 8 lug conversion package from Brothers. I also replaced the master cylinder and prop valve with the ones they recommended. I bench bled the master cyl. installed everything according to their instructions, then bled all the wheel cyl. I have a strong pedal until I start the engine and the booster comes into play then the pedal goes to the floor and the brake light comes on. I called the CPP help desk and they told me to re-bench bleed the master, which I did re-installed and re-bled everything only to find the same issue. The tech dept. at CPP thought I might need a new booster which I didn't replace because it worked before I started this project. I have bleed the entire brake system no less than 4 times with the same results, anyone have any ideas?

[Steve-W]

Originally Posted by Merbob

... I have a strong pedal until I start the engine and the booster comes into play then the pedal goes to the floor

unfortunatly master brake cylinders don't handle that well, even bleeding the brake by pressing the brake pedal instead of using vacuum, can damage the cups in the master cylinder.
my guess is, it's broken if the pedal doesn't stay hard.

[Merbob]

Thanks I'll rebuild it and see if that works

[Steve-W]

and remember, only bleed by vacuum, not stomping on the pedal