Installing a new master cylinder without bench bleeding it first is a classic mistake that leaves you with a spongy pedal you can never fully bleed out. Bench bleeding is quick and worth it. Here is how to bench bleed a master cylinder.
Why bench bleed
A new master cylinder is full of air. Trying to bleed that air out through the whole brake system after install is slow and often unsuccessful; see what a master cylinder does. Bench bleeding removes the air first, on the workbench, for a firm pedal.
The setup
Mount the master cylinder in a vise (gently), fit the bench-bleed kit tubes from the outlet ports back into the reservoir so they are submerged in fluid, and fill the reservoir with fresh brake fluid.
The process
Using a blunt tool, push the pushrod in slowly and let it return, repeatedly. Each stroke pushes air out through the tubes into the reservoir. Keep going until no more bubbles come out and the strokes feel firm - that means the air is gone.
Then install
Cap it, install the master cylinder without spilling, connect the lines, and bleed the system normally; see one-person bleeding. Keep fluid off paint. A properly bench-bled master gives a firm pedal - pair it with quality pads and hardware.