How Can I Inspect Pump

[bilvihur]

Dishes (esp. upper rack) not getting clean. I removed the lower spray arm and found a lot of residue on the inside of the separator screen. I took the pump outlet off, then tried to remove the impeller to get at the separator. I put a 1" box wrench on the impeller and tried to turn the screw with a 1/4" socket, but couldn’t get it to budge, and started rounding it off in the socket. What size screw head is that, and does it loosen counter-clockwise? I can’t see any other way of getting at the separator assembly. Thanks.