# Video: How Brake Master Cylinders Work

Do you want to know how a master cylinder works or how to know if your master cylinder is going bad? Or the difference between a single reservoir versus a dual reservoir master cylinder? In this episode of Summit Racing Quick Flicks, Summit Racing’s tech team explains the function and symptoms of a bad master cylinder.

…on this installment of Summit Racing quick flicks, we are going to help you get stopped safely by answering your questions about master cylinders.

One of the most common questions we hear about master cylinders is simply: how does the master cylinder work? Well, the master cylinder is tasked with supplying hydraulic pressure to the braking system. So when you initially step on the brake pedal the linkage activates a piston within the system and this piston then displaces hydraulic fluid. Once a certain pressure is reached a secondary piston is then activated if you have a dual circuit system. This displaces even more hydraulic fluid, and ideally in these systems, there should be the same amount of hydraulic pressure between the two pistons for the braking system to operate properly.

Another common question we hear about master cylinders is, how I can tell if my master cylinder is starting to go bad? Well, there are a few symptoms you can look for. If your brake pedal feels soft or spongy, or if you press your brake pedal and it sinks slowly to the floor, that’s often a telltale sign that there is a leak inside your master cylinder. On dual circuit systems, you can lose contact between the primary circuit and the secondary circuits within the master cylinder. That is also often due to a leak within the system. In this case, you will just have brake function to two of your four wheels.

People also ask us about single reservoir master cylinders versus dual reservoir master cylinders and basically they just want to know if there is an advantage to using dual reservoir master cylinder versus the single reservoir master cylinder. Quite frankly, the answer is yes and the reason is simple. It’s really all about safety. Up through the mid-1960’s,  most muscle cars came with a single reservoir master cylinder. This single reservoir had serve both the front and rear brakes, so if it failed, all of your brakes failed and you weren’t going to stop. So right around 1967, the U.S. started to mandate that cars come supplied with dual reservoir master cylinders– one reservoir might supply brake pressure to the front brakes and the other supply pressure to the rear brakes. So if one failed, you still had brakes and you still had enough braking to at least get the vehicle safely slowed down.

If you have more questions about master cylinders, brake systems in general, or any high performance related topic, go ahead and leave a question in the comment section below and we will be happy to try to answer those for you.

• Hi Mr. Rebman,

The answer is: Yes. The master cylinder’s front reservoir works the front brakes and the rear reservoir works the back brakes.

Thank you very much for reading!

1. What would cause my left front caliper to lock up? I have new front calipers, pads, rear pistons, shoes, new fluid. And my left front caliper is locking up on me. I pulled the wheel opened the bleeder valve, relieved the pressure closed the valve and hit the pedal and it locked up again. Any idea?

2. Michael Maylor says:

possible flexible brake pipe internal collapse.

3. Sylvia says:

I have a 2011 Tiffin Breeze motorhome which has a brake pedal that doesn’t come all the way back up when I brake. I have to physically lift the brake pedal up each time after applying the brakes. Would this be a problem with the master cylinder or air in the lines, or something else?

• OnAllCylinders says:

Sylvia, this is typically a mechanical issue with the assistance mechanism such as a vacuum booster etc. it is highly advisable to seek the assistance of a reputable repair shop.

4. Mike says:

Having trouble finding a replacement Master Cylinder for my 1980 GMC G2500 1/2 ton 5.7 ltr van, VIN TGL25A4518170, calipers on front, drums on back, mfg’d by GMC Western Pacific, Vancouver BC Canada. Ones I’ve looked at, lines from reservoirs to proportioning valve are larger than one on the vehicle, may be metric, can’t tell for sure. Ideas?

5. David Choquette Sr. says:

I have a car that disc brakes were added to the front and drum on the rear. The MC that came with the car had a drum brake MC, it’s shot. Should I replace the MC with drum MC or go to a disc MC?

6. David Mai says:

My car is a 2012 Honda Civic Hybrid, my brake lights does not light up when slowing down only at a complete stop. The brake pedal is very stiff. Do I have a bad brake master cylinder? Thanks in advance.