You can`t have ANY air in a hydraulic brake system.
Once you have all the brakes bled, with the wheels off you can visually see wtf is going on by have someone press on the pedal.
If the pads are not compressing and releasing you know the calipers are in need of repair, just by loaded calipers and save the hassle.
If the shoes are not expanding then the wheel cylinders are bad, or if the wheel cylinders are dripping.. replace them etc..
If the hoses are bulging under pressure replace them.
Some of those older Chevrolets had brake line problems, rust mostly. If that is the case by the ones custom made for your truck. Unless you have a double flare tool and the time to fuk with it.