mmkevin, .. thanks for your advice. I was just ready to post an update. The problem is solved. I used NAPA 727-2605 O-rings. My shortcoming was not to get the exact dimensions before leaving the NAPA store. We tried a couple of different ones, some SAE, others Metric. But, I am sorry that I did not write down the size of the one I took away. I made the replacement Thursday evening, but Friday was a rainout. I ran it about three house on Saturday, and another two hours today. No oil is coming from the dump valve. Sometimes repairs turn into more costly projects than expected. In this case, it was otherwise. $0.37 worth of O-ring solved a problem, one that I first thought to be a probable pump replacement. LS braintrust proves themselves worthy of acolades once again. Thank you for your help. I learned much in my research of hydro pumps, all knowledge that I hope never to use! If I think of it when I visit NAPA again, I will get the exact dimensions of the O-ring.