You have several problems and air in the lines is not one of them.
Before you start, verify that your brake booster is functioning correctly. Engine off, pump the brakes about a half-dozen times to empty the vacuum chamber. Now, press down lightly on the pedal about an inch and a half and just hold it there. Start the engine. Immediately the pedal should start to drop, as the manifold vacuum evacuates the chamber. This would be normal. Going to the floor is not, but we'll get to that later.
Next,slip your foot sideways off the pedal, and it should return to it's parking spot, probably making a nice thud. Reach down and pull up on the pedal, it should not move. If yours is behaving like this then it is fine and we can move on.
Advisement
Keep in mind that the calipers always drag to some degree. The only way that the pads retract is thru seal-retraction, which is the Square section seal inside the caliper doing it's job.But they should only drag very lightly.So if you can physically turn the wheel, with only a lil effort, the dragging may be normal, so you can skip Page-1
Page-1
#1 is the sticking calipers; you have to solve this first. This has nothing (probably) to do with your crappy pedal, but you have to fix this first.
This problem can be either; hydraulic, mechanical, or operator error;
First the hydraulic; the fluid is not returning to the M/C. It's just that simple. But the why of it is not so simple.
1) As already said, this may be a residual valve.
2) As already said, you may have plumbed it wrong; the frontmost reservoir gets plumbed to the rear brakes.
next the mechanical;
3) the pistons are stuck in the bores
4) the hoses are internally collapsed
5) the square section "o-rings" in the caliper are not functioning properly
6) debris inside the line at a critical juncture
then operator error;
7) the M/C pushrod is too long and the piston is not releasing the fluid back into the reservoir.
hunting;
Of these; 2,3,4,and 5 are an easy test; 1 and 7 are a lil messy; and 6 is a turd so leave it for last, you probably will find the problem elsewhere.
As to
#2, just go look,lol
#3,4,5,and 6; you can do all at once. Dismount a caliper, one atta time, and slowly push the piston back in with a big channel-loc pliers; not a clamp. If it moves,obviously it ain't seized, and the hose is good, and the fluid went back to the reservoir, pretty much ruling out debris..If it feels springy as you clamp and release, then the rubber rings are probably Ok. You may be able to see the piston moving in and out as you clamp and release, just a few thousands of an inch. You don't have to bury the piston in there, stop after you verify the O-rings are working. Re-install it and pump up the pedal. Then check the other side in the same way.
#1, you'll have to go look
#7, I don't think this is your problem so skip it for now.
Ok by now, you should have discovered your problem.
Notes;
The combination valve has nothing to do with this, if you have plumbed the front brakes to the NON-PROPORTIONING, TOP end of it. The top side is nothing but a distribution block.
Page-2
I'll tell you what;
After 4 pumps,you do get a hard, non-spongy,high pedal,
From this behavior we can deduce a few things;
A) That it is not spongy,indicates that there is NO air in the system.
B) After the 4 pumps, the pedal remains hard. This indicates that there are no external leaks.
C) As to the 4 pumps; this seems to indicate that the M/C is pumping out a lot of fluid to make it hard.
As to A), no action is required.
As to B) verify no external leaks
As to C), Assuming the M/C is plumbed correctly, This has to be a fault in the rear system........ cuz the front is stuck ON. So;
Remove the rear brakes all of it, and clamp the w/c pistons into the w/c's, so they cannot move, and the clamps cannot fall off. Now go step on the pedal. It should be hi,hard,not spongy, and on the first stroke; I mean where can the fluid go right? But if it is not high and hard, and still takes more than one pump, and there are no external leaks;......... think about it. Yes, the M/C is bypassing fluid internally, and so is defective.
That's the best I can offer