[dsraven]

the adjuster rod on the master cylinder is for pedal free play. you need some of that to ensure the master cylinder piston comes all the way back into the end of the master cylinder bore when the pedal is released.try checking/doing this
1)-ensure ALL the pedal linkages and pivot points are clean, free moving and not worn out. diss-assemble it all and clean/lube everything.ensure the return springs etc are all there and the pedal positively returns to the top of the normal pedal stroke as soon as the pedal is released.
2)-ensure the master cylinder piston is coming all the way back into the end of the bore. check this by physically looking at it. with the pedal linkage off that should be fairly easy.
3)-check all the brake hoses and steel lines to ensure there are no rusted areas of steel tubing or cracked rubber lines. flex the rubber to see if crackes appear. check all the brake friction linings to ensure they have ample material left and the inside of the drums are not all rusty or grooved out etc. also ensure the linings are bonded or rivetted well to the metal part of the shoe. bonded lining have a habit of coming un-bonded over time ot with excessive heat. ensure the backing plates are not worn out where the shoes ride/rub because that can cause the shoes to want to stay in those worn through holes and want to not return properly. a dab of brake lube on the friction areas of the backing plates where the shoes rub, as well as the contact areas on the pivots and adjuster will also help to keep things moving properly. don't get lube on the drums or the friction lining surfaces.if this happens clean it off right away with some aerosol brake cleaner.
4)-check/diss-assemble the park brake and ensure it is all free moving, not worn out and when released the shoes/cables/linkages move back to their released position right away. replace cables, pivot bushings, clevis pins etc as required. when all good loosen the park brake off all the way so when the brakes are adjusted you don't have any issues with the park brake giving a false "drag" feeling on the service brakes.
5)-adjust all the brakes with the wheels off the ground. tighten them up tight then back them off a bit so the wheels will spin. then step on the brakes to centre the shoes in the drum and re-adjust everything.
6)-adjust the park brake.

a lot of guys will disregard the park brake adjustment because they figure they "never use it anyway". in reality, the park brake is what adjusts the wheel cylinder end of the rear brake shoes "out" to fit the diameter of the drum. if you just forget to do it or don't think it is even required then you will likely encounter a low pedal. because it takes a bunch of brake fluid, and so a bit more pedal travel, to push the wheel cylinder pistons out, so the shoes contact the drum, the pedal will be correspondingly low.

make sense?

here is another tip. if you have a single acting brake master cylinder consider replacing it with a dual acting system. with the single system, if there is ever an issue with fluid leakage, you lose all the brakes (except the park/emergency brake) at the same time. with the dual system you would only lose the front or the rear system, depending on where the leak is located, so you would still have some brakes to get you to the shop or the side of the road at least. I think they make a direct replacement master cylinder to fit those years. it would require a bit of steel brake line work. to see if you have a single or a dual system just look at the master cylinder you currently have. if it has a single steel line coming out of it then it is a single system. a dual system would have 2 steel lines.