You might check the brake pressure reducing valve (PRV) located below the heater box. I have occasionally seen problems with these. I would map the layout of the lines and then test drive the vehicle to the point that the rear brakes lock up. Next, loosen the line at the MC that feeds the rear brakes via the pressure reducing valve. If there is pressure at the MC, you may have a misadjusted pushrod or something wrong at the MC. If there is no pressure at the MC, loosen the output line at the PRV that feeds to the rear brakes. If there is pressure here, you probably have a stuck PRV. If there is no pressure here and the rear brakes are still stuck, work your way back inspecting all steel and rubber lines for kinks and damage and loosen every fitting until you locate the point which pressure is present. This should help narrow things down to find the restriction.