I've done all of 2 dives with my new mask, and it leaks like a leaky sieve ..

It only happens when I look up and the water enters the top of the mask. Having examined it closely, it seems as if the top strap isnt quit tight enough.
However, The strap is as tight as it will go, so I need to move the sliding buckle attachment to the 2nd hole on the top strap. Has anyone else had to do this?
Secondly, do you have to strip the unit to clean the purge valve or is it just a case of flushing it through with fresh water every now and then??
TIA