What brand of valve covers are you using???
When we installed the cast aluminum valve covers (I believe they were the M/T covers) they leaked... We installed 7 different types of gaskets on them including the fel-pro neoprene ones and they still leaked...
We talked to some others at the Mpoar Nationals that weekend and they told us that those valve covers are junk... We changed to a different brand and they sealed first time...