Someone please confirm or deny my theory or at least point me in a different direction.

I converted manual drum front brakes to power discs this winter.

Up until today, I've driven the car a few times and had a low, spongy pedal, which I traced down to air in the lines to the rear wheels. Bad enough that the brake light would come on if I hit the pedal to the floor.

But I must say, I could definitely feel the new power booster working correctly when I drove it. There was for sure a pedal advantage with the car running vs. when it was off.

Today, I bled the rears again and now believe that I've got all of the air out.

BUT, now with the car running, I get no power brakes. The pedal is higher and harder than before, brake light does not come on, but no noticeable difference with the booster hooked up or not. The only thing I did was bleed the rears.

The booster manufacturer recommends 20" vacuum at idle. The best I can do is about 15" with my cam. But I've read where some guys are running as low as 12" and still have power brakes?

My theory is that when I had air in the lines and spongy brakes, the booster had enough vacuum to compress the air and give me some pedal advantage. Now that I've got the air out, there is more back pressure resistance against the pedal, and I don't have the vacuum to overcome it.

Does that make any sense ????????? Or am I missing something else?

The kit was the high performance MBM with 9" single Delco style booster and 1-1/8" bore master cylinder.