How To Test A Brake Master Cylinder

25 May 2017
 Categories: Automotive, Blog


If your brakes are not working properly, check the master cylinder. As the name implies, the master cylinder controls all brake functions. It controls the brake pressure as well as stores the fluid when the brakes aren't in use. Testing the master cylinder is essential for safety reasons. It is easy to check the master cylinder by following these tips.

Prepare to Work

For this project, you will need a flashlight, brake fluid, a clean rag, eye goggles, and gloves. Refer to your vehicle's manual when you don't know where a part is located. Observe the 'check engine' light, which detects problems with brakes in modern vehicles. 

Brake fluid problems often cause soft brakes. A normal brake should remain firm while in idle. Press the brake until it stops, then keep pressure on it. Brakes that drop to the floor under pressure indicate a leaking master cylinder seal..

Check the Brake Fluid

Turn off the engine, let it cool, then prop the hood. Look for the brake fluid reservoir, which is commonly to the rear of the engine bay. Reservoirs are commonly covered by a plastic cap, and some vehicles may use a metal reservoir that has a clamp. 

Unscrew the cap on the reservoir, or pry the metal clamp off with screwdriver. Manual transmissions, or straight shifts, will have two reservoirs. If it has two, remove the cap from the largest reservoir. 

The brake fluid should reach the fill line, and it should not be dark brown or black. Dark brown or black fluid indicates contamination. Add more brake fluid, or replace the fluid, if needed.  

Change the fluid quickly so the fluid doesn't get contaminated with oxygen. Use a flashlight to look around the reservoir for gunk or wetness. 

If the fluid level looks alright, and there is no gunk or wetness present, get someone to press the brake. Watch the reservoir for swirls, leaks, and bubbling, which indicates the master cylinder is defective and needs replacing. 

Inspect the Brake Lines for Leaks

Metal brake lines can rust, which causes a leak through the O-rings, or rubber rims. If you do not see any visible leaks, take a rag and run it over the brake lines. To fix a brake line leak, tighten the nut with the fuel line wrench.

A clean rag indicates there is no issue. Otherwise, leaking brake lines mean the issue isn't with the master cylinder.

Start the engine, and test the repairs. If you don't trust your skill to fix the problem, or you are unsure of what the issue is, take the vehicle to a mechanic.

For more information, you will want to check out a website such as