I have a heritage trailer, and I have 4 wheels drums. I have to replace the master cylinder. I want to install a reverse lockout. I took the factory (I think) master cylinder. And it had a line from the reverse solenoid to a tapped hole in the top of the master cylinder, I assume for fluid run off when it was placed in reverse. SO here is how it ran, it was master cylinder, with a reverse solenoid, to a brass plug with that tiny ass hole, then it has the brake line attached to the brass plug. HOW does fluid go through that tiny hole, and actually make the brakes work? When I took it all apart there was absolutely no fluid in the brake lines, but the master was completely full. Which doesnt shock me cause even fluid couldnt run through that hole. So I bought a new master cylinder, which is composite matterial, and a new trailer buddy reverse solenoid, with only one wire. How do I ground this solenoid, since it only has one wire to run to the backup lights?? ITs suppost to ground to the master cylinder, but its not metal..

Most if not all drum brakes are what is call freebacking. They do not work in reverse. The pressure in the brake system can be from 1000-1600psi. It sounds like the old master cylinder was not pumping any fluid. If you use the solonoid you will have to install a ground wire somehow to it. Maybe solder something onthe brake line.

i will ground through the btake line to the trailer frame if theirs no rust..

You should not need a reverse solenoid if you have the correct brake plates and they are designated as "free backing"

You need that fitting with the tiny ass hole. It dampens/slows how the quickly the brakes are applied and released. Without it, the trailer would overbrake when you slow the tow vehicle. It's a tuning mechanism.

If you take a drum off and spend 1/2hour looking at the mechanism, you'll see what the guys mean by free-backing. Drum brakes, when applied in forward rotation, wedge the shoe against the drum, applying more pressure than the hydraulic pressure itself. They kind of self activate. That's why they need strong springs to bring them back upon release. That same geometry effectively pushes the shoe away from the drum when they are applied in reverse. So even if hydraulic pressure is applied in reverse, the shoes don't have a lot of pressure against the drum.

This is one reason why old cars got away with drum brakes with no power assist (the self applying nature).