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So you don't have "two" air-packs" correct ?
Then I would look at both rear hoses as being bad. It's rare to have both hoses go at the same time, but this is an old truck.
Are you positive the front brakes are not locking up too ?
Does this truck have dual air-packs ? The right side air-pack on a dual system operates the front brakes and the left side air-pack operates the rear brakes.