How to Bench Bleed Your Master Cylinder

Get the Air Bubbles out of Your Brakes

If you install a new master cylinder, you'll have to bleed your brakes; there's no way around it. Bleeding a master cylinder can seem like it takes forever. The pros use strong vacuum pumps to suck the air bubbles out of the braking system quickly, but we don't have these at home. Without it, you have to use the pump pump pump method, a long process and sometimes unsuccessful, meaning you have to do it all over again.

There's a way to avoid a drawn-out brake bleeding session—bench bleed your master cylinder. Bench bleeding means that you are bleeding your master cylinder on your workbench, removed from your car.

What You'll Need:

  • Fresh brake fluid
  • Workbench or table with fixed vise
  • Strong wooden or plastic dowel (rod)
  • Optional: Bleeding kit that came with your master cylinder

Secure the Master Cylinder in a Vise

Master cylinder clamped securely in a vise
Clamp the master cylinder securely in a vise.

Tegger/Matthew Wright

You'll need steadiness to bleed the master cylinder. A bench-mounted vise is a great tool for the job. If you don't have one or don't have a bench at all, you can buy a clamp-on vise that can be removed when you're done. Just don't use the dining room table!

Mount the master cylinder on the vise, holding it by one of its dog-ear mounts. Be sure to mount it level so the air will leave and the fluid will refill properly during bleeding.

Prep With the Bleeding Kit

The cheap bleeding kit included with a new master cylinder
The brake bleeding kit isn't necessary, don't worry if you don't have one.

Tegger/Matthew Wright

Many replacement master cylinders come with a cheap bleeding kit. It consists of two rubber hoses and two temporary plastic threaded inserts.

If you opt to use the kit, you'll screw the threaded inserts into the output ports on your master cylinder (the ports on the side of the cylinder). Then attach the rubber hoses to the inserts and proceed to the next step.

If you don't have the bleeding kit or don't feel like using it, don't sweat it. You don't need it because there's another simple solution.

Bleed out the Air Bubbles With the Kit (Or Without It)

A plastic rod being pushed to activate the master brake cylinder
Depress the cylinder with your rod or dowel.

Tegger/Matthew Wright

If you decided to go with the bleeding kit included with your replacement master cylinder, place the open end of the hoses into a receptacle to catch the brake fluid.

Be sure the reservoir is filled before you begin bleeding. You'll be using your wooden or plastic rod to push the cylinder in (the same way your brake pedal pushes it in while driving).

Now you're ready to start pumping. The trick is that you have to pinch the tubes closed every time you let the cylinder come back out.

So, you'll push in, pinch the lines, let it out, let go of the lines, push in, pinch the lines ... and so on.

You'll see lots of air bubbles coming out of the tubes with the brake fluid, and you'll see bubbles in the reservoir floating to the top.

Be sure to keep the brake fluid topped off in the reservoir. If it runs dry during pumping, you'll have to start over.

If you aren't using a bleeding kit the process is the same, only instead of pinching a rubber tube, you'll cover the holes tightly with your fingers when you release the cylinder. Keep pumping until there are no more bubbles floating to the top of the reservoir.

Replace the Little Caps and Screw the Top Onto the Reservoir

The green caps that come with a master brake cylinder
Insert the caps that came with your master cylinder to seal in the fluid and keep air out.

Tegger/Matthew Wright

When you have pumped the cylinder until no more bubbles float to the surface of the reservoir, carefully replace the little caps that your master cylinder came with. Do this carefully, but don't fret if a small amount of fluid drips out. Screw the top onto the reservoir, and your new master cylinder is ready for installation.