if you cant get it to show any compression than there will be no compression to "leak down".....sounds like its evacuating just as quick as it builds.
rings,hole in the piston,junk cylinder wall,cracked cylinder,valve not seating,tipped valve,head gasket,cracked head....the list goes on of possible things it could be.
your only TRUE bet to find out was is wrong is to pop the head off and see if anything stands out at you.
