If your pushrod is adjusted too short, it will take a lotta pedal travel to get adequate fluid out
If you pushrod is too long, and the power pistons do not park properly to open the C-ports, then eventually the pedal goes lower and lower.
The only thing that cause your front brake fluid to return to the M/C is the Seal-retraction.
The thing that causes your rear brake fluid to return, is the Return springs.
The front brakes do about 85% of the stopping so IMO, you need to start there.
So;
On slider-type front calipers, they slide on the bracket-ways or on the pins. If the caliper does not slide, the brake pressure will bend the steel backer, until the friction hits the rotor, but will not make full contact and so you will be trying to stop the vehicle with very little pad area. The steel backers will now act like springs, and the pedal will seem hard, but maybe the pressure gets all used up in bending the pads. Sometimes the pads get stuck in the grooves ,in the ways, that got there mostly by rattling; you gotta get rid of the grooves. I hard braze them full, then file it all back into shape.
If your caliper has slider-pins; they have to be smooth, dead-straight, and tight in their rubber o-rings, but they gotta be able to slide. I lube them with pure silicon grease; but I take them out first and clean the O-ring grooves, which have a tendency to rust up, which squeezes the O-rings tight enough to stop them from sliding at regular intervals.
As to used calipers or high-mileage calipers, they have square-section O-rings and dust boots. But after years of service, the "O-ring Grooves" rust up the same way and this negates the Seal-retraction, and so the pads tend to drag, which hears the fluid which can with a closed compensating port , lock up the wheel, or if the Port is open, then just glaze the pad which then loses it's power to create friction. If it only happens on one side, the car will pull to the other side.
To prove the C-port is working, have a helper pump the pedal about 3-times and on the last pump hold the pedal down. You, pop the cover off the M/C and watch the fluid by shining a light into the reservoir. Now comes the important part, have your helper very slowly begin to let the pedal up, very slowly. This action will allow the fluid to return, but, if there is any air in the system, it will unload the fluid in one push, which can produce a 3foot geyser that can hit the underside of the hood. This is bad news because brake fluid melts paint. If this happens to you, put the lid back on, and clean up the mess right away. Use plenty of brake-cleaner, and then soapy-water followed by a hot-water rinse. Without air in the system, you will only get a little roiling in the bottom of the reservoir that feed the front brakes.
However, after three pumps, the rear shoes have gone out as far as they can, and when the helper releases the pedal, the return-springs are gonna work hard to push the fluid back to the reservoir, so you could still get a gusher in that reservoir. The key to success of this operation, is to release the pedal very slowly...... especially the last inch or so as the C-port opens.
So what does this prove?
Well if you only get roiling in both reservoirs, then your C-ports are opening, and the pushrod, if nothing else, is at least Not too long.
If you get no gusher in the reservoir that feeds the front, then there is no air in those lines.
If you get a gusher in the reservoir that feeds the rear brakes, this could indicate air, or it could just be the return springs returning to their parked position. To determine which it is, just crank your rear adjuster in hard so the shoes cannot move, then retest. No gusher now proves no air.
See now, I have told you about the possibility of ruining your paint-job, so I will NOT buy you a new one.
Finally, as to your rear shoes; In all likelihood the radius of the new shoes will NOT match the radius of the drums. If true, that means when the shoes go out, they will only be contacting the drums in a couple of tiny windows. They will need time to wear in. If your drums are used, and were not machined, then, they may Not be round, and May be Bell-mouthed; both of which will reduce the rear brake performance, even more.
Below is a cross-section view of a dual M/C.. Study it to see how it works.Sorry it is so small, but it was one of the best I could find; just upsize your screen a bit. There are a few things I want you to notice:1) the frontmost reservoir feeds the rear brakes, and 2) there is a hydraulic chamber BETWEEN the the two power-pistons, and it HAS to be full of brake-fluid for the brakes to properly function. The bench-bleed procedure usually gets the job done. 3) the little springs in there are to return the pistons back to their parking spots, to open the C-ports to recharge the lines, and to be ready for the next stop to feel exactly like the last, with a high and hard pedal. If your pedal is not parking up high under the dash, you gotta fix that, on account of those little springs are not able to compensate for a stiff or seizing pivot-pin.
Ok I gave you almost everything I got so
Happy HotRodding.
Ok now hang on; lemmee tell you something that just occurred to me from a quick look at the drawings.
If you studied it you will notice that the rearmost piston slides right on by the discharge port until the frontmost piston become hydraulically locked and can go no further. Then it starts to push fluid out to the calipers.
Say your rear shoes were seriously out of adjustment. That front piston might just bottom in the M/C bore.
The rear piston would NOT start to pressurize until that front piston has stopped, with no regard as to why it stopped.
You would of course, have a very low pedal.
So now, the rear brakes would hardly be contributing and all the braking would have to be done by the front calipers. If simultaneously the front pads are Not contacting the rotor properly, but the steel backs are acting like springs ...... yeah I can see that requiring a two-foot, max-effort attempt, and scary results.
