But its really odd how I can ride the bike around using only the front brake lever and I can ride for miles this way and nothing happens. But the minute I touch that rear brake pedal it locks up.
Yes, that's the type of discovery that leads to determining where the fault lies.
When I cracked the bleeder loose for the center piston (rear brake master cylinder) nothing happened. When I cracked open the bleeder for the two outer pistons (SMC system) the brake fluid came right out under pressure.
And that is the proof that the SMC hydraulic circuit is causing the lock-up of the rear brake.
I imagine that when you disassemble the SMC, you will find that the piston is bound within the cylinder bore, with a fair amount of crud at the open end of the bore. When the SMC piston binds in the cylinder bore, the piston primary seal ends up blocking the compensating port, which prevents the return of brake fluid from the rear brake caliper.
When the front brakes are applied with a bound up SMC piston, the SMC is unable to 'push' brake fluid to the proportional valve, and then on to the rear brake caliper (for this discussion we can ignore the proportional valve's role in the brake circuit). So when no brake fluid is 'pushed' to the rear brake caliper, then no brake fluid can be 'trapped' in the rear brake caliper, and the rear wheel remains free to rotate.
However when the rear brake is applied with the same bound up SMC piston, brake fluid is 'pushed' into the SMC cylinder midway between the piston primary seal, and the piston secondary seal. As the rear brake continues to be applied, brake fluid is 'pushed' past the piston primary seal via the recuperation grooves, through the proportional valve, and on to the rear brake caliper, where the rear brake caliper functions correctly.
The problem arises as the rear brakes are released, the brake fluid that had been 'pushed' from the SMC out to the rear brake caliper is now 'trapped'. This is due to the SMC compensating port being blocked by the bound up piston primary seal. As the compensating port is the only return path for the brake fluid, the brake fluid is effectively 'trapped". With the brake fluid unable to return, brake fluid pressure remains steady, and the rear brake caliper does not release. With continued use of the rear brake, and the heating up of the 'trapped' brake fluid, it won't be long before the rear brake 'locks up' completely. Note that the blockage of the SMC compensating port by the piston primary seal may not be total, which is why the rear brake may release after a period of time as the brake fluid pressure slowly dissipates.
Brake fluid passing by the piston primary seal via the recuperation grooves, but failing to pass back in the opposite direction, is similar in concept to how air passes by the leather piston seal of a tire pump. As the pump handle is drawn up, air easily passes around the leather piston seal, then as the pump handle is pushed down the leather piston seal resists the air trying to pass, and instead pushes the air into the tire.
Perhaps
sketch of the SMC is in order.
I think it's time to order those parts now.