residual valves aren't needed to 'keep the shoes against the drums', the adjusters do that, the shoes can't move any further away from the drums than the cylinder pistons allow, but some drum brake systems do incorporate a type of residual valve in the MC to maintain enough pressure to keep the cylinder cups sealed, keep a firm pedal feel, and prevent drawing air in if there's a leak .... yes, the residual valves could be naks problem
Bill
