Master Cylinder for 72 Chevy Stepvan

[bradleysfoodtruck]

I have a 1972 C30 Chevy Stepvan. Not sure who made the body. It's got a weird brake system that no one I've taken it to can figure out. The current mechanic says it needs a new master cylinder but he can't find one. There is no part number on the current one, and any one he tries it not the right size. Does anyone out there know anyone who can help? This is a food truck and the season is starting already so I really don't want to remove the MBC and send it off for weeks. Am attaching pictures of one of the boosters so you get an idea of the whole system.