Testing your brake master cylinder is a good step towards diagnosing potential braking problems in your automobile. The brake master cylinder serves to create pressure in the hydraulic fluid that powers the brake system; and when it is not working properly, the necessary pressure is not being made. This reduced brake function is very dangerous and should be assessed and repaired as quickly as possible in order to avoid a possible accident.
Open the bonnet of your car and locate the brake fluid reservoir. It will be towards the back of the engine bay and will be a plastic cylinder filled with hydraulic fluid. If you have a manual transmission car, there will be two of these; the brake fluid reservoir is the larger one.
Have someone else sit inside your car and apply pressure to the brake pedal as you observe the brake fluid reservoir. If you notice fluid swirl or bubbles forming in the reservoir as they press on the brake, then your master cylinder is not functioning properly and will most likely need to be replaced.
Inspect the area around the master cylinder for fluid leaks. If you notice any brake fluid leaking out of the master cylinder, then it is not functioning properly and will most likely need to be replaced. However, if the fluid is leaking out of nearby brake lines or tubing, then your master cylinder is probably not the problem.
Apply pressure to the brake pedal until it comes to a stop and then hold the pedal there, sustaining the pressure. If moments after the brake pedal has come to its initial stop it begins to drop down again slowly, then the master cylinder is not functioning properly and will most likely need to be replaced.
Determine if the brake master cylinder is the problem. If there are no fluid leaks at the master cylinder, no swirl or bubbles in the brake reservoir, and the brake pedal does not slowly descend with constant pressure, then the master cylinder is working properly.