My research indicates you would have drum brakes on the rear. If so make sure that the wheel cylinder on the passenger side is operating smoothly. When a cylinder starts to leak the brake fluid picks up moisture; rusts the steel wheel cylinder; and gums up the piston causing the piston to stick in the cylinder. That “thunk” you hear is probably brake shoe forcing the piston to retract.
So do an inspection on both rear wheel brakes. See if there is a rust gunge behind the forward cylinder boot. If you find a problem with one you should probably replace both as well as the shoes if they have significant wear on them. Also look at the drums to see if they needed to be replaced or turned to clean up any rust or grooving there.
Also you have a proportioning/load valve actuated by the position of the rear axle. This sends more hydraulic pressure to the rear brakes as more load is placed on the rear wheels. There could be a problem in this valve that is sending too much pressure to the rear brakes.