The pressure switch can be removed at any time, doesn't seal anything. If the O rings were leaking, fluid would be flowing from switch...
There are seal repair kits for the pressure differential valve (Torino, Mustang etc) but I have no idea if same kit fits full size.
OK, that's encouraging.
I will first try to soften the rubber using 1. the interweb method: soaking in silicone and sealing in airtight bag for a week. 2. modifying the switch connector. After looking into that "rubber renue" it appears to be geared toward rubber plattens used on printers to make them soft and "tacky" again. I want just the opposite, soft and slippery. As for the kits: yes, I have seen them. I have the old pressure diff. valve out of my car, and think it probably can be rebuilt. The switch is available no problem, but the pigtail connector is nowhere to be found. I doubt if I am the only one having this problem. Or else no one else goes to the trouble of redoing everything in their brake system like I have done. But to be fair, the pr. diff. valve from the X-100 was still fine. So maybe the switch to DOT 5 in my car back in '95 was a bad idea. Erik said he has seen old DOT 3 brake systems ruined by DOT 5 in the past. I guess it just depends.
Just took the Marquis out for a drive today, since the roads are clean and dry although it is almost freezing. I wanted to see how the car acted with the new fuel filter. Much improved! It was badly hesitating before, now it is virtually gone!
The brakes are good and confidence inspiring. Makes me wish Spring would hurry up and arrive.