What is the purpose of the master cylinder?

Answer 1

It is like a syringe, the needle used when giving a shot. Fluid is in the cylinder and the clutch or brake pedal pushes in and forces brake fluid into the lines and then to the brakes or clutch. It is a closed system with fluid in all the lines so the pressure from the pedal is transferred to the brakes or clutch.

Answer 2

To try to make it simple, your typical vehicle has a master cylinder and four slave cylinders, one for each wheel. With the help of your foot, the master cylinder sends pressure via hydraulic fluid though lines to the slave cylinders which inturn operate the brake linings to do the work of stopping the vehicle.

Answer 3

The master cyl is like a syringe, and there is a piston which is pushed by the brake pedal that forces that fluid out and into the brake lines and down to the calipers which squeeze the rotors. When you release the brake pedal the pressure returns to normal and the fluid returns back into the master cyl. The replenishing port is to top off the cylinder of any fluid which does not return. The reason some fluid does not return is because as the brake pads wear and are pushed closer to the rotor, it's the brake fluid that fills this void and that brake fluid comes from the master cyl. So without replenishment the brake petal would go further and further to the floor as more and more fluid remained in the brake calipers. Make sense?

Or picture a syringe filled with water, except the syringe has a small hole on the top side that is fed water from a small tank above. Now picture when you depress the plunger and squirt some water out the tip, it's gone, but when you pull the plunger back water is drawn in from the tank above refilling the syringe for another round. That port in the top side attached to your tank is your replenishing port. Brakes don't use much fluid at all per push of the pedal because wear on the pads is so small, but whatever teeny bit remains in the calipers as the pad wears is refilled by the tank above thru that port. You might drive a car for 50K miles before needing to add more fluid to the tank. Also, when you install new brake pads you'll need to force the fluid filled calipers back to accept the new thicker pads and this forces all that used fluid back up into the master cyl, up thru the replenishing port and into the tank above. If you've added fluid fluid during the time the pads were wearing down, there is a good chance that the tank will over flow when the original fluid is returned. Brake fluid is nasty stuff and will remove paint, so when installing new pads be sure the tank does not over flow. Either scoop excess out or use a paper towel to soak it up.