Have you thought of using a in line shut off valve like used in the stock air condition cars(1969 Dodge Polara) that have a cable to control them
I have one like those in my stash of spare parts to use in a street car build soon, most parts store should have them in stock or order one, many cars used them
I think the upper rear nipple is the return hose
I would install it in the hose from the front lower nipple