3000 spitting fluid out the PS res

Possibly air in the system if it was dry.

Possibly air getting into the system through dry rot hose connections on the supply side.

The first should clear up with a few minutes of running and cycling the steering full turns a few times.

The second requires replacement of the hose plus the same action as the first.

Are you saying that it overflowed enough that it's only half full now, or were you running it half full to begin with? If you were running it when it was only half full to begin with, then that may be the reason, as the pump will put bubbles into the fluid if it starts sucking air, and then the fluid expands as it foams up. Once you get the tractor running again, I would try filling it to the full mark, starting it and cycle the steering wheel from full stop left to full stop right and back again a couple of times, and then check the level and top it off again if needed and then try it like that. If it still comes out the cap I would replace the filter next as a clogged filter can cause the fluid to foam as well.

Thanks guys, I'll fill it to the specified level cold before doing anything else.