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I made a plate to replace the mc lid. I used 1/2 inch alum with 1/8 rubber gasket, then used a c-clamp to hold it on . Threaded it for 1/4 inch pipe. Put a ball valve on it and a qd on the othe end. Hook up your air hose with about 20 psi. Start at the farthest point from mc and work you way to the closest.
Everything I've read from GM says purge from the caliper closest to the master cylinder and work to furthest away. Does not matter if you do the front or the rear system first. Purging air in this manner causes the air pockets to travel futher away from the MC.