OTOH, the RABS valve was apparently not the cause of my sinking pedal. The system is tight, I've gone through everything checking and replacing as necessary. I tested the booster with a vacuum pump, it held 23 inHg leaking down to about half in 15-30 minutes. I've never suspected the master until now since it looks new and doesn't leak. I guess I'll be installing a new mc, I can't think of anything else that I've not checked or replaced...