I recently looked at the brakes on my friend's 2011 F-150...LF pads were down to the base plates, RF pads were gone and part of the rotor was stuck to the base plates, rear pads were half wore...that told me the struts were shot, and verified that rear shocks were leaking, so she was effectively down to that LF wheel doing all of her braking, which freaked her out a bit as I had predicted this weeks before just by eyeballing the wheels: LF covered with brake dust, RF covered with rust, rears no brake dust at all.
After replacing the front rotors and pads, I had her pump the pedal while I opened the bleeders to flush the dark brown brake fluid out. Those rear calipers did not release much fluid, but after finagling with'm using a prybar, they started creaking when she applied the brakes and fluid began to flow. After she drove it for a few days, she reported back that the odd braking behavior had disappeared, which is what I was thinking in that the rear calipers were sticking before I fiddled with'm but were actually functional now. The proportional valve was probably sticking too, but probably started to work again once those rear calipers started moving.
What was restricting that flow at the rear bleeders? I assumed that debris had accumulated for years in the caliper that effectively locked those rear cal.pistons in their bores, also clogging the bleed port. I did not have time to verify this as she was on a schedule, but driving the truck on the local lumpy roads after I presumably knocked those sticky pistons loose may have freed up the works.
I'm not familiar with your Toyota MC, but if the brake pedal is applied in your application, does that MC port fully get closed from the reservoir? If it is partially open when you are applying the brake, then no pressure will build at the brakes.