If you've just rebuilt the master cylinder on your vehicle or you've purchased a new or re-manufactured one and are about to install it, you're going to want to bench bleed the master cylinder. This step will save you time and energy since the master cylinder is the heart of the hydraulic brake system. If there is air in it, it's going to take quite a while to work its way through the system. Bleeding the master cylinder will save you time and money down the road.
Place the master cylinder in a bench vise tightly and securely so that the reservoir is level and not tilted.
Attach the master cylinder bleed kit to the master cylinder outlets. If you're using the supplied kit, the fittings may be plastic. One of the fittings should be threaded and the other will have a hose adapter for it. Clear hose is recommended to bleed brakes so that you can see the fluid and whether it contains air bubbles. Some of the cheap kits that come with the master cylinders may supply nontransparent colored rubber hoses that make it impossible to see making them a bad choice for bleeding a master cylinder.
Slide the hoses onto the fittings. You may have to twist them back and forth a little to work them on.
Trim the hoses with the utility knife leaving enough length to submerge them in the fluid of the reservoir. You don't want them too long, however. When you apply pressure to the piston, these hoses are going to want to dance around while purging, so keep in mind that you're going to have to hold them in place or secure them somehow while you bleed the master cylinder so they do not become exposed to the air. If you are using someone to help you pump and bleed the brakes later, you could recruit them to hold the lines while you prime the master cylinder.
Fill the reservoir with the proper DOT-approved brake fluid suitable for the application. Having someone hold the hoses in the reservoir or holding them with one hand yourself, take the large Phillips head screwdriver and press the piston in slowly. Allow it to come back and continue to do this in full, slow strokes. You're looking for the fluid to no longer be aerated through the hoses and no longer have air bubbles in the master cylinder. Even though you're recirculating the brake fluid, if you pump the piston slowly and smoothly, the bubbles will dissipate. Give it 15 to 20 strokes and check the fluid for aeration. When all the air is out and you've finished bleeding the master cylinder, it is ready to install in the vehicle. Keep the master cylinder bleeding kit installed until it's mounted to the brake booster, and you're ready to insert the steel brake lines. Remove only one adapter at a time and attach the steel brake line immediately to minimize spillage and allowing more air to get in.
Bleed the brake system when the master cylinder is installed, filled and the fill cap is secure. This will purge any air that has contaminated the hydraulic system while the master cylinder was removed. Check to see if there's a specific brake bleeding sequence for the vehicle, but a rule of thumb is to start with the line farthest from the master cylinder (which would obviously be the right rear wheel) and work your way to the closest (left front wheel) last. If you have a vacuum brake bleeder or power brake bleeder, then hook it up and open the brake bleeder screws one at a time with the drain bucket below to capture the purging fluid, until no air is purged from the system and the brake fluid appears lucid.
If you do not have a vacuum or power brake bleeder, get an assistant behind the wheel and have him pump the pedal four times and stop while holding the brake pedal down and then open the brake bleeder screw manually with the hand or line wrench. Remain at each wheel until the fluid becomes lucid and then move to the next. Check the master cylinder often during the bleeding of the lines in sequence. If it runs dry, then you've just purged air back into the system and will have to line bleed all over again. Check 6 or 7 times after opening the brake bleeder screw or before you move to another wheel, whichever comes first. Always remember to top it off and replace the fill cap. Wipe any wet areas down with a shop rag.
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