[dsraven]

on the brakes:
I see a firewall mounted master cylinder so assume you have done a check to ensure a full pedal stroke yields a full master cylinder stroke as well. if you haven't done that check then start there. a low pedal during set up may result in not enough stroke of the actual linkage to yield a full stroke of the master cylinder.
some info on the rear axle brakes would be good. we could read back through your thread but that takes time. last I seen you had "thrown" and axle under it to move it around. anyway, disc or drum back there?
-adjust the pedal and linkage to the master correctly so you have free play at the pedal and also at the pushrod to the actual master cylinder from the booster. sometimes these seem to get mismatched at the supplier so just ensure the pushrod length of the booster is correct for the master you are using. if the pushrod is too long it won't allow the master cylinder piston to retract far enough to get a refill of fluid when the brakes are released and can result in a low pedal
-if you have drum brakes anywhere on the unit ensure they are in good shape and adjust the service brakes with the park brakes backed off, then adjust the park brake. this will adjust the star wheel adjuster end of the brake shoes to fit the drum first without the park brake/wheel cylinder end of the shoes interfering with that, then the park brake adjustment will adjust the wheel cylinder end of the shoes so the wheel cylinder piston stroke will be less and there will be a "higher pedal". step on the brakes after, set the park brake a couple times and release, then retry the adjustments. sometimes guys will adjust the drum brakes but not apply the brakes during this and in some cases the shoes drag on the drums because they don't get a chance to recenter themselves. also check the backing plates for grooves where the shoes rub against them. a groove will allow the shoe to drop into the groove and not be able to come back out with the shoes return spring pressure. this keeps the shoes off center as well so hard to get an adjustment.
-do yourself a favor and bench bleed the master cylinder until no bubbles come out of the bench bleed hoses. you may have to walk away for a bit because air in the fluid tends to stay in suspension for a bit so you end up putting that bubbly fluid back into the cylinder if you don't let it settle out. the other option is to put the bench bleeding hoses into another container and just keep filling the master after each stroke of bench bleeding
-ensure the ports of the master are connected properly to front and rear according to the supplier. if disc front and rear ensure you have the correct master cylinder with no residual valve in the port to the rear brakes. the master cylinders are different for disc/drum and disc/disc. drum brakes have the res valve pressed into the outlet of the master usually. also ensure the master has the correct piston bore for your system. a small bore will yield less fluid delivery per stroke but easier to push pedal. a larger bore will deliver more fluid but less brake application pressure given the same pedal effort. also a "higher pedal" under the same circumstances because it delivers more fluid per stroke.
-fill master reservoir, place lid on but don't tighten up so it can vent easily
-if no knowledgeable helper is available pick up a one man bleeder kit or build one yourself and set it up on the bleeder screw that is furthest away from the master cylinder. understand how the one man bleeder works and try for a nice tight seal on the hose to bleeder screw. don't unscrew the bleeder too much or air can get sucked in around the bleeder screw threads.
-open bleeder screw on the rearmost, furthest away, wheel (measured in brake line/hose length). bleed brakes until clear, no bubbles in fluid.tighten the bleeder screw securely
-go to next furthest away bleeder screw, do the same. ...and so on for the rest. ensure you check the master to keep it full so you don't suck any air in when you release the pedal. if you do suck air in, start over from scratch on the bleeding.
-does this yield a brake application that is not spongy and within a normal range for pedal travel?