Air hissing only under load accelerating seems air compressor and boost related

Those older engines that got air from the pressure had what was called a boost (or turbo, can't remember which) protection valve. Find a diagram of your model air dryer, you'll find it.

 

Just a side note, I see Van Horn in one of the previous post. I highly recommend them. The Owner is a great guy. Helped me out of a jam once, when no one knew what the problem was. He could have easily towed my Truck and charged me a fortune. I’d never have known different. They no longer work on Trucks, but have a huge assortment of parts. Things others don’t carry. Very helpful and honest. Specialty tools also. I’ve yet to spend any real amount of money there. I plan on it though, as they really earned my Business.

 

Only because it happened once to me. Lol. Glad it’s an easy fix.