The PDWA (C37217; RTC2525;Girling 64048953) is a safety device, much like seat belts. Sorry to hear you’re having difficulty resetting the valve. Here are some pictures of an original valve and it’s components. Hope it helps. Corrosion inside the valve is usually the culprit. It’s the result of improper brake maintenance. The ROM is so-so in the reset operation. I"ve found it’s best to remove the valve, overhaul it, reinstall and then bleed the brake system. I know it’s a lot of trouble but the only way you’re going to know what’s going on IMHO.
There are two pistons within the valve body. Each is fitted with a square o-ring to hydraulically seal the unit. The right piston has a groove in which the small steel ball resides. The ball groove keeps the switch from making contact. If a leak develops the associated pistol will move causing the ball to ride up on the piston, actuating the switch which turns on the BRAKE light.
The spring in the center is fitted to the switch (C37468; RTC-826A;Girling 6467738).
Girling made a Service Kit - SP2615 but as of late, it’s not been available. The kit comprised two square seals, 2 copper washer and the steel ball. Maybe one of the major suppliers will see fit to re-introduce the kit! The switch was used on numerous British vintage car PDWA valves along with Ford Motor Cars.
Without the ground spring the system won’t turn on the BRAKE warning light if actuated. They’re easily fabricated.
PDWA body Girling part number.
With care the o-rings may be reused. They’re square and the material is resistant to brake fluid. Standard o-rings will not work!
If you have not idea when the last brake flush was, when the last time the PDWA was overhauled then it’s time to remove it, take it apart and see what’s going on. It’s 40+ years old and probably never been serviced.
Hope this info helps. Nice stable of cars. Also have several daughters living in the area.
Stay Well and Enjoy Happy Trails,