The usual suspects would be a kinked brake line or deteriorating flex line; possible sticking in caliper pistons; frozen caliper slides. On an old truck like this, any one of those, or even more unusual things, are possible.
Certainly it's time to go up on the rack and pull all the wheels. That's also a good time for a brake fluid flush if you haven't done that in years.
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Certainly it's time to go up on the rack and pull all the wheels. That's also a good time for a brake fluid flush if you haven't done that in years.