more pedal pressure usually indicates a mechanical problem rather than worn brake shoes/pads...
you may have conventional air brakes, air-disc or the stopmaster type, and it really doesnt matter in this case, but it sounds more like a bad "auto adjuster" or an oil leak due to a bad wheel seal...
if one of the auto adjusters, no matter if its a slack adjuster or the internal type like air-disc and stopmaster use, if its worn and not holding the shoes at their proper adjustment, it will take more air to put them against the drum with the proper pressure...
for safety reasons, you need to have it looked at by someone who knows the system, as if one brake isnt working proper, which makes the other work HARDER, if one more should fail you could be left without enough brakes to stop the unit..