I sent you a PM. I do not know BOSS plows. Their's may not even be called a c valve. If you want me to I will edit the title of your post to make people aware you are looking for help regarding a BOSS pump. I think most of your responses now will be for a Meyer like the info I gave you.
Karl - To help make it easier to answer your question, is it possible to provide a bit more info? Any idea how old the unit is - a quick look at BOSS' website/service centre/technical information/power units/power unit history shows Monarch power units used from '81 to '84, and another make from '84 to '88.
Also, is the unit acting up and if so, what are the symptoms?