Fought the same problem for 7 years but with front disc conversion and rear drums. Could never lock up the front brakes no matter how hard I tried, rears would lock up a little bit. I have a factory 9” booster car with 15” of vacuum. I replaced the booster a few times, no change. Added a vacuum canister, no change. I finally purchased a 9” dual diaphragm booster and installed it 2 weeks ago, front wheels locked up at 30 mph panic stop but after a trip around the block the wheels started dragging. Removed the push rod between master and booster, shortened it by 1/8 of an inch and drove again with an even better result. The front brakes now work and feel exactly like my factory power front disc 73. Two weeks of driving with working brakes, I couldn’t be happier. I hope this helps someone.
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