Just a thought- If you're bleeding brakes by yourself, with a hose running into a jar of brake fluid... when the bleeder screw is loosened, a small amount of air can leak past the screw threads and into the wheel cylinder/caliper when you let the pedal back up. Years ago I had a problem with this, and now I get a helper to work the pedal. I attach the bleeder hose & run it into the jar, then have them press lightly on the pedal. THEN I open the bleeder screw. The fluid comes out under pressure, and I close the screw BEFORE they let the pedal back up. This keeps air from getting in around the bleeder screw threads. This works for me, I hope it is a help to you!
In general, if the pedal is soft, there's something in the system that has 'give'. Air bubbles are of course a prime culprit. New shoes (rear drum brakes) might not fit the drum precisely, and have a bit of 'springback' until they wear in. I would really go after air bubbles, & check the way the cylinders and calipers are oriented & where the bleed screws are placed- is there a way that air could get trapped in a 'pocket' so that it cannot bleed out? (I once had a car with a hydraulic clutch- the bleeder screw was on the BOTTOM of the slave cylinder- When I took it loose and turned it upside down (with the hose still attached) so that the bleeder screw was on the TOP.... THEN I got ALL the air out of it! Hydraulic cylinders and metal lines don't 'give' appreciably, and flex lines also don't 'give' unless they are REALLY old & in bad shape.
LOOK- EXAMINE- THINK- Figure out what is going on. DON'T blindly replace parts!
All the Best!
Jim | 
