Problem 1st started with right rear tire skidding under brake presure. Now brake light on after changing wheel cyl. Have bleed lines, light still on. How to center proportioning valve, tried all the sugestions in repair mannuals. Replaced some rear lines, flushed resivor thru system with 3+ quarts of new fluid, and continue to get air bubbles. Replaced master cyl (bench bleed) and problem persists. Any suggestions?

Thanks,
Mike