ah, the genuine article! A true proportioning valve or differential valve with the switch that will make your brake light come on if there is lack of brake fluid or air in the line. Does your '65 have a brake light switch on the dash? If so, and the brake light won't go out after you've bled the entire system, I know how to get it out 99% of the time. When the valve shifts, it shunts brake fluid to the "good" side, the side that has no air in the line~ a safety feature. If you don't have a brake light on the dash, and you go to bleed your brakes and you can't get fluid on either the front or the back, the valve would have shifted and needs to be brought back to equilibrium.
I installed the adjustable ssbc proportioning valve instead of the factory valve.
I had the same problem on my '57 chevy Bel-air booster install, and had to drill the hole like you did. Don't feel alone
dne'
The pic on the left shows the valve has shifted, one on the right is normal operation.
