Letmgrow gave you good advice on checking for line pressure, I have the same brake setup as you on a 1995 P30. A few other things to consider;
Did you replace the hardware and return springs on the rear shoes when you replaced them? There is a few psi of residual pressure in the rear brakes to keep the brake cylinder pistons and push rods in contact with the shoes. If the springs are stretched and weak this can contribute to the problem. Make sure the right spring is in the right position. Make sure the leading and trailing brake shoe are in the correct locations.
Are your drums out of round? The drum brakes are self energizing any drag from out of round drums amplifies a bit and will build heat.
Not a common problem but the brake combination valve (AKA proportioning valve) that maintains the residual pressure in the rear drum brakes and balance the pressure between the front calipers and rear wheel cylinder can malfunction. The combination valve is about 12 ď to the rear of the MC inside of the frame rail.
There is no residual valve in the master cylinder on the P30. I can only guess that rear piston in the MC hanging up on return could cause this problem.
As for the booster to MC rod clearance. I doubt that one because my logic would say if the clearance is too tight both front and rear brakes should drag. But I could be wrong. If it were my bus I would loosen it like LetMGrow says but I would insert a couple of thin washers between the mounting faces, retighten and test the brakes.
If you go swapping boosters keep in mind that rod lengths vary between models and years for the same booster.