How To Bench Bleed Your Master Cylinder

Installing a new master brake cylinder also requires that you know how to bleed your brakes, but bleeding a master cylinder by yourself is no easy feat. Professional mechanics have strong vacuum pumps that will suck the air bubbles out of the brake system quickly, but an ordinary person won't have one at home.  Without a vacuum pump, you will have to pump out the brake master system manually, a long process that takes several tries before you can do it right.  But there is a way to avoid wasting your time trying to manually bleed your master cylinder, and this is by bench bleeding your master cylinder.  Bench bleeding is a way of bleeding the master cylinder by removing it from the car and doing it on a work bench before reinstalling it.  Here's how to bench bleed your master cylinder.

  1. You will need fresh brake fluid, a strong wooden or plastic rod, and a work bench or a table with a fixed vise.  A bench-mounted vise is the most important tool for the job because it will keep the master cylinder steady.  If you don't have one, you can buy a clamp-on vise and install it onto a table.
  2. Mount the brake master cylinder onto the vice by holding it by one of the dog-ear mounts. Make sure it is mounted levelly so that the air leaves and the fluid refills properly during the bleeding process.
  3. Most replacement master cylinders come with a simple bleeding kit made out of two temporary threaded inserts made of plastic, and two rubber hoses.  If you want to use the kit, you need to screw the threaded inserts into your master cylinder's output ports. These are the ports on the side of the cylinder. Attach the rubber hoses to the inserts, and place the other end of the hose onto a receptacle in order to catch the brake fluid.  But if you don't have the bleeding kit, don't worry - you don't actually need it.
  4. Make sure that the brake fluid reservoir is filled before you start bleeding. Use the wooded or plastic rod to push the cylinder in, the way the brake pedal pushes it in while you drive.
  5. If you have the bleeding kit, you're ready to start pumping. The trick is to pinch the tubes closed each time you let the cylinder come back out.  The motions are: push in, pinch the lines, let the fluid out, release the lines, push in, pinch the lines, etc.  You will see several air bubbles come out of the tubes along with the brake fluid, and bubbles will also be floating on top of the reservoir.
  6. Make sure the brake fluid is topped off in the reservoir.  If it runs dry while you pump, you'll have to repeat the process.
  7. If you don't have a bleeding kit, the process is essentially the same, except you have to cover the holes with your fingers instead of pinching the rubber tube. Keep pumping until there are no more bubbles.
  8. With the air bubbles gone, carefully replace the caps on your master cylinder.  Screw the top back onto the reservoir.
  9. 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. Your newly bled master cylinder is now ready for installation.


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