It should be common knowledge that you must bench bleed a new master cylinder before installing the master on the vehicle. Traditional bench bleeder kits use brake fittings and copper lines that direct fluid from the outlet ports on the master back into each reservoir.
We recently had the chance to try a new Wilwood master cylinder on one of our cars and the master came with a set of plastic fittings and clear plastic hose. The time-honored bench bleed process is to put the master in a bench vise and slowly depress the pistons to pump air out of the piston assemblies for both reservoirs. What we noticed was that toward the very end of the bleeding process, a small number of air bubbles would only travel a portion of the way up the clear plastic hose and that if you quickly released the plunger, the bubbles would be sucked back into the chamber(s).
We discovered that if you pumped the pistons to the bottom of their travel and then held the piston fully depressed, the air bubbles would eventually travel up to the top of the arc of the plastic hose. Then if we very slowly released the plunger, the bubbles would remain in the top of the plastic. Then in the next pump action, the bubbles would be forced into the reservoir. This purges the last air from the master cylinder which will improve brake pedal feel.
Wilwood sells this clear plastic brake bleeder kit separately and it’s an inexpensive kit that can be used with most master cylinders to help with bench bleeding. The Summit Racing part number for the kit is WIL-260-11593.