proportioning valve used as a shutoff?

[WSSix]

Crazy idea time. Can I use my proportioning valve as a shutoff valve while I work on my rear brakes? I'm trying to prevent the fluid from coming out of the master since it will be sitting a few days while I do the work.

Which I guess brings me to another question. Do they make plugs and caps that I could screw onto the different fittings in the future while I do brake work?

Thanks