Sometimes after installing a fresh brake master cylinder into any type of car, the pedal will often repeatedly go to the floor and it may take many cycles of pedal pressing and bleed screw opening & closing before reservoir fluid even begins to slowly trickle it's way down into the master cylinder. Priming the master cylinder with fluid before installation eliminates much of this. Although there are several ways of doing this, "bench bleeding" the master cylinder is the "classic", tried and true method and it requires a minimum of new tools.
Installing a new master cylinder is helped with a bench bleed to ensure all the air is removed from the unit. BUT you do not have to do the bench bleed if you don't want to. You can still do the bleed with the master cylinder on the vehicle.
Before bench bleeding the master cylinder, completely install all of your replacement or upgraded brake components, including the master cylinder and brake lines; this is so you can hook up the master cylinder as soon as possible after it has been bench bled, to keep gravity from leeching fluid from the master cylinder during the installation of the other components.
After successfully rebuilding a master cylinder (or when fitting a new one) it is a good idea to bench bleed the master cylinder before installing it in the car. This will fill the master cylinder with fresh air-free fluid and in effect "prime" it for integration with your car's hydraulic brake system, making your on-the-car brake bleeding a little easier.
The basic idea is to create mini hydraulic system on your bench. You can use old brake line fittings if you have them or you can purchased a master cylinder bleed kit from your local auto parts store. A new master cylinder may include the necessary parts already. The kit should consist of a number of plastic fittings which are designed to fit in the outlets (usually two, front and rear) of your master cylinder. One end of the fittings will be threaded and the other will have a round smooth hose adapter. Thread the appropriate fittings into the outlets on your master cylinder. The kit will also contain a length of plastic hose. My kit had black hose but I found some spare clear hose and used it instead - this will allow viewing of the air bubbles passing through the hose. My kit also had a plastic clip used to hold the two pieces of hose together and clamping to the edge of the fluid reservoir.
Clamp the cylinder firmly in a bench vise so that the cylinder area is level. If it is pointing upwards the air will remain in the cylinder. Slide the hoses onto the fittings. Cut the hoses just long enough to reach into the reservoirs and remain submerged - the shorter the length of hose the better. Place the other ends of the hoses into the fluid reservoirs (you'll probably have to hold them in place somehow because once you start pumping they will want to flail around in the air and spray brake fluid everywhere). If you can get a helper that is ideal.
Fill the reservoirs with new brake fluid, and pump the piston slowly and evenly, full strokes. Use a big Phillips screwdriver because its tip doesn't damage the piston and the handle gives you something to lean against. I would not worry about the fluid getting recirculated because it is brand new and you are creating a temporary hydraulic circuit with the hoses which will not become contaminated with dirt. The air which is still in the system at this point will be bled out. Pump the cylinder until the tubing contains no more air bubbles and no new ones emerge from the master cylinder on the down stroke. On my master cylinder this took about 15 strokes, some may require more & some may require less. Keep going until the air stops as this will make the task of bleeding the brakes in the car much simpler. When all the air is out, mount the cylinder in the car. Here you have to be careful to prevent the fluid still in the hoses from spraying your car and any other painted objects nearby - brake fluid is a great paint remover! If you decide to remove the hoses before installing on the car, make sure to plug up the fittings - I just held the hoses up while transferring from bench to car. Once the master cylinder is mounted in the car, remove the fittings and connect the brake lines. You'll lose a little fluid but the check valves in the cylinder should stop any major leakage. Now you are ready to bleed the brakes in your car and it should be a lot easier than if this step was avoided.
Or you can purchase a vacuum bleeder pump kit (about $40 in discount auto-parts stores), which can be used for both clutch & brake hydraulic systems, as well as testing vacuum hoses. The choice is yours. You can either build a bleeder kit or buy a vacuum bleeder pump kit. Actively pulling (vacuum) or forcing (pressure) fluid into the brake system are alternative methods to bench bleeding, but they require either an air compressor or a source of vacuum.
