[Killer Bee]

have you inspected all four wheel ends to make sure they're all adjusted properly and making good shoe contact?

flushed and bleed the entire system? brake fluid is hygroscopic so needs to be replaced periodically.. not every year but more often than every 50 years..

does the master cylinder hold or does it slowly creep down?

I see a lot of folks converting and upgrading systems that would probably be sufficient if properly maintained..

my '65 C20 in all its farm years of road salt and manure will still lockup all four wheels..

as far as increasing your pedal effort, I'm sure there are kits for vac and hydro if you're truck has p/s..

good luck!