Jump to content

How does a master brake cylinder work...

Recommended Posts

I'm rebuilding the brakes on my BRDM-2 and I have stripped the master cylinder (and removed all the sludge...).


I'll be sending the cylinder off to pastparts to get the pitting removed.


In the meantime, can anyone explain exactly how the cylinder works. I assumed it would be just a piston in a cylinder with hole which is exposed when the piston is fully returned for the fluid to return to the reservoir.


The BRDM cylinder is exactly like the diagram below, except it has a compressed air booster.


So why are the recuperation holes in the piston there? Why is the fluid on both sides of the main seal (primary cup)?


Both seals move with the piston, the secondary seal (A) is attached to the piston and the primary seal (B) is pressed against the piston by the spring.


There's a one way valve at the left hand end of the spring.


ASE 5:Brakes|Braking Systems|Braking system components|Master cylinder





A) (blue seal) secondary cup

B) (blue seal) primary cup

C) (opening for brake fluid) compensating port

D) (opening for brake fluid) inlet port

E) (opening for brake fluid) recuperation small holes

F) (grey rod) push rod or rod from brake pedal

Link to comment
Share on other sites

This is how a basic simple hydraulic system works, but May not necessary be the same as yours, the system must be completely filled with hydraulic fluid. It is essential that all air is excluded from the cylinders and pipes; once this has been done the following state will exist.



  • · Each wheel cylinder centre compartment and pipes leading back to the master cylinder will be full of fluid.
  • · The compartments in front of and behind the head of the piston will be full of fluid.
  • · The reservoir or the canister will be filled up to about two thirds of its total capacity.


Brakes Applied.



When the pedal is depressed, the piston moves forward, compressing its spring and covering the small port in the cylinder. The pressure of the fluid opens the delivery valve, and fluid is forced into the pipe lines and wheel cylinders, thus moving the plungers away from each other and applying the brakes.

During the movement of the piston the compartment behind the head remains full of fluid because it is in permanent communication with the reservoir and a cup washer at the rear prevents fluid escaping to the atmosphere. It looks like your push rod in your diagram is the adjustable type?



Brakes Released.



When the pedal is released the piston in the master cylinder is forced back by its spring and this movement is quicker than the return of fluid from the wheel cylinders to the master cylinder (this is due to the piston spring holding the return valve closed until it is overcome by the brake shoe return springs. The resulting suction in the front of the piston will draw the edges of the cup washer away from the piston and fluid will flow from behind the piston through the small holes and over the cup washer to the front compartment.

The powerful action of the brake shoe springs will draw the shoes away from the drum, thus forcing the fluid from the wheel cylinders back through the pipes to the master cylinder which now has an access of fluid owing to the recuperating action. The excess is returned to the reservoir via the small return port in the cylinder. This also prevents pressure building up in the system as a result of the expansion of the fluid when heated. The piston spring is set to maintain a static pressure in the system at pre- determined pounds per square inch. If the fluid expands it forces the return valve of its seating and pressure is released through the return port.

Link to comment
Share on other sites

  • 4 years later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...