If your water system was ever subjected to freeing temperatures when not drained, you may have cracks in the plastic housing. I never had any issues with my valves in my 2007 Tour Master, but pretty sure it never got frozen either.
Similar manabloc valves are made by the company in the link below named Viega...maybe they have something you can substitute.
https://www.amazon.com/Viega-50250-2...a-653531718639
Chuck