Bench bleeding is done to get all the air out of the master cylinder before installation.
Easily done by making a short length of brake line with the correct fitting for each output port on the master cylinder. (some new master cylinders come with plastic fittings and lengths of rubber hose to bench bleed)
If you know a mechanic, he/she probably has a few of these plastic fittings and rubber hose in their toolbox.
The procedure is to clamp the master cylinder level in a vice (Careful not to clamp on the cyl itself if you can help it, use one of the mount bolt flaps) attach the short brake lines to the outputs and bend them up so they end up inside the reservoir. The ends of the hose/line have to be under the level of the brake fluid after you fill the master cylinder reservoir to work right.
After you have the reservoir filled use a Philips screwdriver to slowly push the piston into the cylinder repeatedly until no more bubbles come from the tube ends under the fluid level or any ports in the bottom of the reservoir. CAUTION... When you start to pump be careful of brake fluid spurting up from the ports in the bottom of the reservoir, brake fluid is nasty on the eyes and skin ; ]
After the air is all out, plug the two outlets and install in the vehicle. Put the lines on with minimal fluid loss when ready to attach.
You still have to bleed the brakes complete at each wheel but it makes it much easier.
Especially important to bench bleed master cylinders which do not sit level in the vehicle...
Good luck
