Ah ok,,, so you have disk on the rear now. On the check valve you should be able to use any check valve with the same size line fittings as the line you have. If the fronts aren't working then there must be a high spot in the line somewhere trapping air. I would imagine you have new pads on this now? I was going to suggest you reverse bleed the calipers. Just squeeze them so all the fluid flows back to the reservoir. Then pump them up again.
Also you should have about 1/8" clearance between the master cylinder and the booster push rod. Then 1/8" between the pedal and the booster. If the push rod hits the master then you are not getting a full stroke.