To clarify you are wanting to put an exhaust brake on a farm tractor? What brand of engine? I have seen Jake brakes on Cummins and Cat engines on Case has them on the new Iveco engines for the hills in our area. The old Steigers were common to have them as not great brakes. They are set up just like a truck with the master switch and three speed controller on the panel and a switch on the engine throttle to activate when you pulled back the throttle all the way. I'm not even sure if there was one on the clutch pedal because you never touch the clutch when you got going with a power shift transmission. I ran a Steiger several years with a Jake on a Cummins engine pulling a Yielder drill around the hills. It was great breaking over the hills and turning on the Jake and holding everything back and not having to use the brake.

As another had said though not to be used if slick as you still need brakes because if the wheel currently breaks loose and starts spinning backwards its going to do it more without using brakes at all relying on the Jake than if you have your foot on a pedal applying brakes. However there are several brake setups that are used on tractors and you don't have brakes usually at each corner like a truck.

If you have a newer tractor engine with electronic fuel then you may have an additional problem it may not be programmed for an exhaust brake so it will take some research to see if it can be done. It sounds like you have started the process and if all else fails a bigger tractor will be more capable of holding back a big load on the big hills. Good luck.