I had a problem on a '68 GTO like this that drove me crazy - even had a new reman'd booster on it. Could not get the tires to lock up and almost felt like manual brakes by how much pressure I had to put on the peddle. As last resort I went up under the dash and where the connecting rod comes out of the booster there was a felt pad or filter around it to keep junk from being pulled into the booster as the diaphragm moved back and forth. After removing it and hitting the brakes it almost put me thru the windshield. I think the filter was acting like a plug and not letting the booster do its job or allowing the diaphragm to move. Easy to check from sticking your head under the dash. This "tip" has helped out several folks in the past.
Hope this helps.
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