‘66 Barracuda Brakes Suck — Proportioning Valve Maybe?

“Was there also a change in linings? How are the MC lines connected to the prop valve?”
They are linings with good, dry material left, I didn’t need to change them. The front line from the MC goes into the factory check valve, then into the proportioning valve. (You can spot the check valve in the bottom of one of the pictures, it’s about the size of a small cigar.). Then it goes straight to the rear brakes.

“Are the rears locking up under hard braking at higher speeds?
In what way does it not stop well?”
The rears will not lock up at 25 or even 45 mph. It takes forever to haul this machine down, as bad or worse than my ‘64 did, and it had 9” drums all around!

“Can you crack the bleeds on the rear cylinders and get fluid when someone presses the brake pedal? If not, then something is blocking flow.”
Yes, fluid comes out when the rears are bled. They don’ appear blocked.

“Rear brakes do have a lot to do with initial braking. Check the adjustment, check the shoe placement (or post photos) and check the linings.”
I compared the rear brakes to the ‘66 FSM and they are identical. I wiggled the drums off no problem yesterday. The wheels spin with a slight drag like they’re supposed to.

“If the problem is more related to slowing down after the initial application, and the above all checks out, then focus on the front brakes.”
I’m not sure what to check in the fronts. With the front in the air, the wheels spin, then with the pedal down, the wheels stop spinning. Perhaps the pucks aren’t coming all the way out.

“Tandem master's front outlet should feed the rear line. Only one residual valve is needed. Whether it's the one in the Master's outlet or the 66 in-line one from the single outlet master doesn't matter.”
I bled and bolted in the MC, but did NOT mess with any residual valves.

Thanks for brainstorming for me, Mattax.