There is no need for either in a single circuit brake system as the components are matched for displacement. You would only need a proportioning valve if something has been altered, like changing over to a different caliper or altering the rear wheel cylinders. When going to dual circuit brakes and changing the master cylinder you’d definitely want a prop valve. I’ve never seen a check valve or residual pressure valve on any Brit car.
Warped front brake rotors will push the pistons back into their bores resulting in a “dead” pedal and requiring several pumps to get brakes again. I don’t know how air got into the system only in the back, but I’d check to make sure all of the brake pipe fittings and the bleeder are tight and that the connections for the flex hose are tight. if they are loose, air will be drawn into the system as the brake pedal is released.
