Got a power conversion w/ front disk rear drum from right stuff detailing. Pedal is firm and brakes seem to work when engine off. When car starts pedal sinks and brakes barely catch just before the floor.

I ran engine, turned off engine, waited 5 min. Pulled booster valve and it hissed, showing that the booster holds vacuum.

I bled brakes using a vacuum pump and pulled a lot of fluid through. Any ideas?

Try adjusting the rear drums tighter. That should make the pedal higher. The brake pedal is supposed to drop slightly when the car starts. That's one of the tests to be sure the power booster is working.

I'm pretty sure the rear drums are adjusted correctly. I'll double check though.

I know the pedal should sink some, but not all the way.

I would get help from a friend and bleed it the hard way. Tried bleeding mine using the open valve and let it drip method, some guy on youtube advised. What a joke. Not familiar with the vacuum pump method though, maybe itīs better idk.

did you ever find a fix? i'm having the same type of problem but recently switched to 4wheel disc brakes....any idea?

for whatever reason it seems to be the norm to have trouble bleeding the back brakes. make sure the calipers are mounted with the bleeder straight up, many of the aftermarket brackets don't this correctly. I had to take my calipers loose and tilt them back. I ended up running a whole quart through the rear brakes.

If the brake booster has failed, the brake pedal would be hard even with the engine running. Try pumping the pedal with the car off and holding it, if the pedal starts to fade to the floor your master might be by-passing internally. If all is ok so far check the length of the booster rod(between master and booster).

Did you bench bleed the master cyl before installation? Real hard to get the air out if you didn't. Might try bleeding with the back of the car as high in the air as you can get it, which tips the master cyl down.
HTH, Jim

Did you change the porportioning valve when you did the conversion I missed it when I did mine the valve is smaller and your fronts may not be grabbing properly