What is this thing called?

I’m building my skylark and am trying to get familiar with everything but I don’t know what this thing is

 

Do you have a shop manual?

 

Looks like a nice, clean engine compartment!

 

That does look clean. What are you building on the car?

 

Square thing (master cylinder) is a hydraulic cylinder that pushes brake fluid down the line to the wheels and activates the breaks when you push the brake pedal. The big round thing (power brake booster) uses vacuum from the engine to make it easier to push the brakes to stop. Without that, it takes a lot of force to press down the brake pedal. On modern cars, the master cylinder usually has a plastic see-through reservoir and a plastic lid that holds the brake fluid.

 

Serious question or leg pulling - on any other forum this question would have drawn no end of ridicule.

V8buick guys are a differnt breed!

Bob

 

some kind of vacuum thingy that helps with binding

 

If you’re (re)building a car that is apart or missing parts, keep in mind you may not have power brakes. If your car was built with manual brakes, the square master cylinder will bolt directly to the firewall-you won’t have a big round vacuum booster. (I’m guessing the photo you posted isn’t the car you’re working on).
Patrick