[dwcsr]

if you get a very low pedal with the rear connected its either air, rear hose is ballooning , the drums are to thin and flexing, or all of the above.

looking into the port of the master you should see the check valve if its there. it will look like a steel ball or a plastic piece. Take a paper clip and unfold one leg. Insert the paper clip into the port you suspect and if it only goes a 1/4 inch or less the it most likely has a valve in it. if it goes a majority of the way in it does not have a valve in it.

I would go to a 1.25" bore. It sounds to me you just don't have enough volume to move the rear cylinders the require distance. If you pump them with the residual valve in them and it raises the pedal temporarily the bore is the issue