I have a set from 1988 in the box next to my PC. Changed them last year when the aux water pump went south, but the valves are still o.k.
I have no more direct info about the repair but the old one are repairable and there should be repair kits.
In such case just check on my website what is available.
Here a direct link to Johan's write up
http://shrubbery.student.utwente.nl/...tervalves.html