Did you replace the valve seals when the rebuild was done? If you did, did you slide the seal on the valve using a thin piece of cellophane? If you just slid them on the valves, you probably nicked the seal.

Always lubricate a seal and protect the seal lip when installing them. It takes a little more time but a lot less time then doing it over.
<img src="/forums/images/graemlins/kewl.gif" alt="" />