I have 2 Straight trucks that had beds on them, a 94 peterbilt 379 and an 88 357, both have 20 k front axles and 46k rears. They each only have the single yellow dash valve. It would be useful for one of the trucks to work as a Dump truck and be able to tow a tag trailer, and set the other one up to work as a tractor. Neither of the trucks has ABS. I have seen a kit for this, am I missing a major road block here? and does the dump truck have some other considerations to keep the braking ability high without a trailer connected?
Back in the bad old days of early 90's, there was a thing called {bobtail proportion}, tractors could come equipped with a BPR-1 valve. That valve performed the opposite function, unless a trailer was attached then you had full brake application on tractor, unhooked it limited air flow through the BPR-1 valve, a line ran from tractor protection valve to BPR-1 supplying full air to tractor brakes when [red trailer button} was pushed in. To make it work at full application, it could be tricked by running that from tractor protection to direct air supply at a tank. * You don't have that obviously so there is no issue, it would be plumbed as any other to pull a trailer.