SMC valve leaking from dump side

I've got 3/8" SMC valves in my car and have had them for about 4 years now. My right rear dump is leaking pretty badly out of the "out" side of the valve so that corner wont stay up for more than a minute without bumping the switch again.Ive taken the whole thing apart twice now to clean it and look for any bad seals and they look ok to me so im not sure if theres anything i can do to fix it besides buying a new one. Should i replace the seals anyway, and if so where can i get them? SSD used to carry them but since they sold the biz i havent been able to find them.