How to Bench Bleed Your Master Cylinder

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 work bench, out of the car.

What You'll Need:

  • Fresh brake fluid
  • Work bench or table with fixed vise
  • Strong wooden or plastic dowel (rod)

Secure the Master Cylinder For Bleeding

Master cylinder clamped securely.
Clamp the master cylinder securely in place. Thanks to Tegger for taking pics.

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.

The Bleeding Kit

The cheap bleeding kit included with your new master cylinder.
The brake bleeding kit isn't necessary, don't worry if you don't have one. Thanks to Tegger for taking pics.

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 (my preferred choice), don't sweat it. You don't need it.

Bleeding the Master Cylinder

Use a plastic or wooden rod to activate the cylinder.
Depress the cylinder with your rod or dowel. Thanks to Tegger for taking pics.

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. If you aren't using the kit, read on anyway.

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).

If you're using a bleeding kit, 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.

Prepping the Master Cylinder For Installation

The green caps that came with your new part.
Insert the caps that came with your master cylinder to seal in the fluid and keep air out. Thanks to Tegger for taking pics.

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.