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tchevs,
Not sure what proportioning valve you have but on a GM proportioning valve if you remove the brake light switch from the proportioning valve you can reach down with a small flat blade screw driver and recenter the valve and there will be NO brake fluid loss. Attached is a cutaway pic of a GM proportioning valve and a pic of the tool to hold it centered. The tool is plastic and works fine & not expensive. Hope this helps.

Oops, I just reread your post and see you also have rear discs so I doubt you would have a proportioning valve.