Master Cylinder Bleeding Proceedures.
NOTE: DO NOT bench bleed a master cylinder on the car. On a g-body the master cylinder sits at an angle and it WILL NOT get all the air out of the master cylinder. Use a vise to hold the master cylinder level to the ground to bleed the master cylinder of all its air.
I like to use plugs to bleed the master cylinder of air instead of the procedure that uses hoses to recirculate the fluid from the master cylinder ports back up to the reservoir. Why?
When using plugs to close off the ports of the master cylinder, this procedure will let you know if all the air is out of the master cylinder AND if the master cylinder is bad. You don't want to find out your master cylinder is bad after you have it installed and are trying to bleed the rest of the system. You most likely will not get all the air out of the system when your master cylinder is bad. New or rebuilt, it is always good to make sure your master cylinder is in good working order before bolting it onto the car. It will one less thing you have to trouble shoot if you run into other issues when you are trying to trouble shoot braking issues.
Steps to bleeding a master cylinder:
1. Mount the master cylinder in a vise with the bore of the master cylinder level with the ground. Do not use the top of the reservoir as a guide because is may not be level with the bore of the master cylinder. It may be at an angle versus the bore of the master cylinder.
2. Use the appropriate size solid plugs to plug the outlets of the master cylinder so no fluid can escape the ports.
3. Fill the master cylinder with the appropriate amount of brake fluid.
4. Use a rod to SLOWLY cycle the master cylinder piston in its bore. DO NOT use a flat head or phillips heat screwdriver because they have sharp edges and could harm the bore of the master cylinder. I usually use a nut driver that is used for ¼” drive sockets as a rod because the end does not have any sharp edges and there is a handle to hold onto.
5. After cycling the master cylinder piston SLOWLY a few times, the piston should become rock solid and only move about 1/16 of an inch or less down the bore.
6. After the piston becomes rock solid, push in on the master cylinder piston and hold for 45 seconds. If the piston slowly moves down the bore of the master cylinder, you have a bad master cylinder. If the piston says rock solid and does not move, you master cylinder is good.
7. Mount to your car and bleed the rest of your system starting with the brakes furthest away (passenger rear) from the master cylinder and working your way to the closest (drivers front) brake.