Master Cylinder: How to Change

Changing a master cylinder is not hard. If you follow these instructions, you will be able to do it in less than an hour.

The first step in replacing a brake master cylinder is knowing it is bad. This is not very hard. Common telltale signs of a bad brake master cylinder are:
1.)The brake pedal goes to the floor when you hold it.
2.)The RED brake light is on and the brake fluid is still full (also means parking brake is on).
3.)Or there is brake fluid leaking from where it the master cylinder bolts to the brake booster.
If you experience any combination of these symptoms the master cylinder is bad, and should be replaced.

So now, it's time to gather a few tools. A basic wrench and socket set, a turkey baster, a can, a screwdriver, and 1 foot of vacuum hose is all you need.

First get the turkey baster and the can, and open the lid of the master cylinder. Now suck the brake fluid out of the master cylinder with the baster, and put it in the can. Get all the fluid you possibly can out of it.

Now set the can and the turkey baster aside, and get your wrench on the lines. Take them loose. There is no special technique, just turn it counter clockwise. If the line twists, and gets damaged then you absolutely have to change that line. It wasn't going to hold up much longer, anyways.

Got the lines off? Good. Get your socket set and remove the 2 bolts that hold the master cylinder to the booster. Again nothing special, turn them counter clockwise, and they will come right off. It's unbolted! Get it out of the engine compartment.

Now you're ready to put the new one in!

Before you put the new master cylinder on the booster, you need to 'bench bleed' it. It is not hard, and only takes 2 minutes. First, remove the plugs from the line fittings in the master cylinder. Then fill the reservoir about 1/3 with fluid. Put your finger over the holes for the lines, and push the master cylinder with your thumb or a screwdriver until fluid comes out the lines without air bubbles. It is VERY important to hold your finger over those holes when you 'let off' the piston in the master cylinder to let it return back to normal position. Pump it 6-8 times, is usually plenty good enough.. When you get done with that process, put the master cylinder to the booster, rest it on the studs, and go wash your hands. Something you should be VERY careful of is not to get brake fluid on your car. It will eat the paint.

See 15 minutes into the job, and you are already bolting it back up! Get your socket set, and putthe 2 nuts back on the studs, to secure it to the booster. Turn them clockwise, and snug them up pretty good, but not He-Man style. Then get your wrench and tighten the line back to the master cylinder. That's it, the master cylinder is installed!

Now it's time to bleed the brake system.

*A little back ground information*
In order to bleed the system as fast as possible, you have to know how it is designed. Brake systems are plumbed different, depending on the type of vehicle they are on.

Front wheel drive vehicle brake systems are 'Diagonal' This means one of those lines on the master cylinder feeds fluid to opposite corners of the vehicle.
Rear wheel drive, and 4X4 vehicles are plumbed where one of the lines feeds either the front or the back of the vehicle F.Y.I. The line in the front of the master cylinder feeds the back brakes.

Now you know how the lines are laid out. The proper order is to start at the farthest point from the master cylinder, and work your way to the closest. So on front wheel drive vehicles you will start with the back driver wheel, then move to the front passenger, and then do the same for the other corners. On rear wheel drive and 4X4 vehicles, you start at back passenger, then back driver, then the same on the front. That is the order you do it in, for the fastest and best results.

Now you're ready to start bleeding the brakes! We will be done in 15 minutes.

Fill the master cylinder up, and put the lid on it.
Put your wrench on the bleeder screw, and then put the vacuum hose over the bleeder, and lay the other end of it in the can. Open the bleeder screw, about 1/2 turn counter clockwise, and have someone press the brake pedal. Once the pedal is down, close the screw, then they can let off the brake pedal to let it rise. Once the pedal is back up, you open the bleeder again, and repeat the process until clean fresh fluid comes out the hose. When you finish one wheel, refill the fluid, and move on to the next one. It only takes 2-3 minutes to do each one.

Work each wheel in the proper order, and then refill the system to the proper level. Check pedal operation. Is it nice and hard, and stay at the top? If so, it's good.
Wash up, you're done!

See that wasn't so bad, was it?
Plus, you just saved about $300 by not taking it to a shop, and you are back on the road quicker!

By the way, If you need help with anything, just click the link somewhere on this page that says something about talking to a mechanic. You might just get me!

Stay safe! Thanks for reading.