Brakes won't hold pressure
Well I have no idea. Here's what I'd do.
Go downtown and buy some fittings, including some inverted flare plugs. Pull the rear line loose at the distro block and plug that port. See if the pedal "hardness" improves, and it should be absolutely hard. If so, concentrate on something in the rear.
If you conclude that it's "fronts" I would pull the calipers and pads and "rig" a 2x4 or other block in the caliper. Slowly actuate the pedal until you have the pistons out against the blocks. Be sure to monitor fluid in the master so it doesn't "hit air."
Next, take a C clamp on the longest line (pass?) and retract the piston. this will force fluid (and air) back up to the master. You can pull the front fed line off the master, and collect fluid. Use a second clamp to force the remaining piston back in. Keep the hoses at the tops of the calipers. This should create an absolutely air-free pair of calipers.
If that doesn't fix it, then it surely is something in the rears.
Rear shoes have been tightened up.
I would REALLY tighten them up, until the rears simply will not turn
