Sounds like back brakes are the primary culprit here. It was unclear to me if the front brakes were really an issue or not?
Things to look for on the rear:
Emergency brake cable binding?
Automatic adjusters on the rear brakes "adjusting themselves" to tight?
Things that might apply to front and/or back brakes:
Brake pedal binding and holding pressure on the master cylinder?
Pistons in the master cylinder binding and not returning to normal position?
Binding pistons could be caused by:
Oil contamination, which swells the rubber, this would apply to ALL rubber parts in the brake system. Wheel cylinders, brake lines, etc. Bear in mind brake fluid is NOT in ANY WAY "
oil" based. It is compatible with WATER, not
OIL. A small amount of oil contamination would take some time to swell the parts and eventually cause trouble.
Water/Rust/Contamination in general:
Same basic effect as the "oil" scenario above. The very close tolerance parts in the "brake system" can bind/stick with even a little bit of contamination, such as "moisture" being sucked in by the brake fluid. Thus a brake system can "go bad" after simply sitting for a long time and rarely if ever used.
There is ONE great law that ALWAYS applies to vehicles:
If "miles" don't get you, "time" will. Don't drive that many miles? Change your oil ANYWAY every few months! Wether you drive or not, your "stuff" is breaking down, use it or loose it.
Ernie