Should you "bench bleed" a master cylinder and why?
You really do have two choices when it comes to bleeding a new brake master cylinder. Despite when anyone may tell you, master cylinders can be bled on the vehicle. However, if you want to make things easier for yourself, it is best to "bench bleed" a new master cylinder off the vehicle prior to installation.
First, you need plastic master cylinder bleeding hoses and fittings, which normally arrive with a new master cylinder for off-vehicle bench bleeding. If your new master cylinder doesn't have them, go see your local auto parts store for a bench bleeding kit. Install fittings and lines, and loop them back into the reservoir(s). Fill the master cylinder with clean DOT 3 brake fluid (if you're running mineral base fluid).
Depress the master cylinder piston slowly and observe air bubbles. Do this repeatedly until all air bubbles have passed. Add fluid as necessary. Be careful not to depress master cylinder piston too far. When the flow of all air bubbles has stopped, you have successfully bled your new master cylinder. Cap off ports and install master cylinder.
Although most of you understand the damage DOT 3 brake fluid can do to paint, we cannot emphasize this enough. DOT 3 mineral base brake fluid works like paint remover. It will permanently damage your paint. Take extra care to protect paint from brake fluid.
Bench bleeding a new brake master cylinder makes light work of overall vehicle brake bleeding.