I have replaced the master cylinder.
I have replaced the rotors.
I have replaced the pads.
I have rebuilt the calipers.
I have replaced the brake booster vacuum hose check valve.

I bleed the brakes starting from the rear passenger, rear driver, front passenger and then front driver's ......here's my issue.....When I do the fronts, I lose pedal pressure.
I cannot keep brake pressure.
It seems that I'm getting air in the system but where?
Everything is tight no leaks.

Could a bad brake booster be the culprit?

Thanks in advance.

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1976 280C
SLOWER DRIVERS KEEP TO THE RIGHT.
DRIVE RIGHT PASS LEFT