I used these guys up the road in Orange, CA about 10 years ago. They rebuilt my vacuum booster, master cylinder, both front brake calipers and prop valve. I was very happy with their work and customer service. As for the topic of this thread, however, my original prop valve leaked ever so slightly, and I couldn't get it to stop. They sent me one of their new brass valves, and I haven't had any problems since. Maybe not every valve is able to be rebuilt. IDK. But I would use them again in a minute.
http://www.abspowerbrake.com/
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1972 Chevy C-10, SWB, Fleet, 350/350, PS, PB, all stock, Survivor.