George is right (nothing new there).

I just re-did mine maybe three months ago. I did change the valve cover gaskets, but they were not leaking, the BREATHER cover seals were leaking but since I was doing the work, I wanted to be DONE with it when I was done! The breather covers are sealed using RTV compound and leaks from the breathers are very common.

The safest and most effective way to repair them is:
1) Remove valve covers from engine.
2) Separate the breather covers from the valve covers.
3) Carefully remove the old seal compound and clean both surfaces very well. (I used a toothbrush dipped in gasoline, this dissolved the RTV very well. I then cleaned with "Purple Power".)
4) Apply new RTV and put the two halves together, bolts finger tight. Wait 1/2 hour.
5) Tighten bolts to 7 ft-lbs.
5a) Leave assemblies to dry and RTV to set (I left mine in front of a dehumidifier for a few days - but I'm all OCD)
6) Put assemblies back on the engine, replacing gaskets (but since yours are already new, just reuse the new ones).
7) Tighten bolts to 10 ft-lbs.