Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Another possibility could be the rear differential needing lube change and adding the additive for the clutch packs. When I first got my 77 if I went for a drive it started out ok but after a few miles I could feel the rear drag as you describe. had the rear serviced drove around in circles like suggested and it was ok
1970-73 used a distribution Block
Its separate chambers are fed from both the front and rear master cylinder outlets, with a spring-centered spool located in a channel between the two chambers. If there is a fluid leak or air in either the front or rear brake system, on brake application that difference in pressure will shift the spool one way or the other, and the center of the spool will contact the tip of the warning switch; that grounds the switch circuit, which lights the "Brake" warning lamp in the instrument cluster (the same lamp is also operated by the parking brake when it's engaged). With no air or leaks in either system, the spool remains centered in the block due to equal fluid pressure at both ends of the channel.
1974-77 used Proportioning Valve (combination Valve)
The proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached. This is done to prevent rear wheel lockup on cars with light rear wheel loads.
The valve is designed to have a by-pass feature which assures full system pressure to the rear brakes in the event of a front brake system failure, full front pressure is retained in the event of rear failure.
The pressure differential warning switch is designed to constantly compare front and rear brake pressure from the master cylinder and energize the warning light on the dash in the event of front or rear system failure. The valve and switch are so designed that the switch will latch in the “warning” position once a failure has occurred. The only way the light can be turned off is to repair the failure and apply a pedal force to develop up to 450 PSI (3100kPa) line pressure