4020 brake valve

Working on a 4020 for a friend. One of the plungers on the brake valve is froze. How big a deal is it to rebuild that unit? If it's froze will I be able to clean it up to work? Does anyone over reconditioned or worst case do we have get new? Thanks
When the boots go bad it lets water down into the plunger are and the stick,, most of the time the plunger is too rusty to clean up..you can mail me for help,, or ship it to me,, I can service it and bench test it here
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