When you change a brake master cylinder on a Chevy Astro van or any other vehicle, it will cause air to get in the hydraulic system that you must bleed out. You can speed up the process of removing air by bleeding the master cylinder before installing it--a process known as bench bleeding. You will need to do this on a surface where the cylinder can be supported without spilling fluid. The process described may also work for a number of other models.
Insert threaded plugs into the master cylinder's outlet holes--the ones on the sides that the brake lines will connect to. Make sure the reservoir is properly seated on top of the cylinder with its grommets.2
Fill the master cylinder and reservoir with fresh brake fluid.3
Remove one of the plugs from its outlet hole.4
Push the piston assembly on the cylinder to expel air from the cylinder. Place a finger on the open outlet hole, release the piston and wait for fluid to flow from the reservoir to the piston bore--this will take several seconds.5
Repeat the previous step, pressing the piston so it forces your finger off the hole, until only brake fluid comes out the hole, then replace the plug.6
Repeat the previous three steps for each of the outlet holes on the master cylinder.7
Fill and top off the master cylinder reservoir with fluid and install the diaphragm and cover onto the reservoir.