[superbirds]

On my 1975 the rear throttle valve linkage mover had leaks in the rubber seal. Sounds similar to your issue, I also had a continuous leak at the air manifold under the dash part for the parking brake. I found a couple new parts on EBay that fit. They were not both exact but worked great. I think I also bought a rebuild kit for the rear unit. Shortly before I found the new old stock unit. If you need the kit drop me a line I’ll try to find it. I read on a couple truck forums that a lot of truck shops used to keep these on the shelf. I think this in an older system that may not be true now.
Good luck on your project.