I've recently acquired a late s/n 7520, when I first went to look at the tractor the previous owner told me that occasionally when the tractor sits for more than a week the fuel lines drain back and you have to crack the line on the injector pump to prime it before starting the tractor. Not a huge deal but I'd like to try to fix the problem. Looking at diagrams the earlier s/n tractors had an inline check valve installed between the tank and the filters. However the later s/n tractors do not have the check valve and have a different model of transfer pump. Now it'd just be my guess that the newer style pump must have an internal check valve that has failed on mine and the last owner didn't want to spend the $ to replace the pump. I'd think a guy could simply get the old inline check valve and put it in somewhere but i'm also wondering why the previous owner didn't do that. Does anyone here know why that would be a bad idea or any other insights to this issue? Is it possible that the old check is too restrictive for the newer pump? Thanks.