Installing a new master cylinder requires a bench bleed to ensure all the air is removed from the unit
After successfully rebuilding a master cylinder (or when fitting a new one) it is a good idea to bench bleed the MC before installing it in the car. This will fill the MC 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 but I didn't so I purchased a master cylinder bleed kit from my local auto parts store. A new MC 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 MC. 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 MC. 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 hold them in place somehow because once you start pumping they'll 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. I used 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 MC on the down stroke. On my MC this took about 15 strokes some may require more, some 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 spaying 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 MC 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.
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