Hot Rods 57 Pontiac Master Cylinder

Pretty sure that's what's inside this (case) but can't seem to find any info on the case,cover or whatever it's called

 

that's called a "heater"

MC is probably under the floor.

 

Pontiacs used the Treadle Vac booster/master cylinder unit, it's under the floor. You will need to find a brake specialist that can rebuild them unless you have the knowledge yourself. I had to take mine to a place called Power Brake Exchange in San Jose CA.

 

Ahhh, thank you