Number one verify you rebuilt the correct valve. Even the best goof once in a while. On the old seat washer were there grains of sand embedded or were there pits? If you have sand or debris in the manifold and didn't flush it the new seat washer may already have pits in it. Sometimes the brass seat itself is a little rough and will let water seep through. I take some medium grit emery cloth to clean the seat. That works very well. On a rare occasion it can be the solenoid plunger. Very rare in fact. Good luck!
If I can't fix it, it's broken!