OP, how fast does the tank drain? 1 hour, 1 day, 1 week? Can you hear the leak?
There are two obvious leak sources with most compressors. One is the outlet and on that compressor there likely isn't a shut off valve so you could be leaking out of the regulator. If the regulator is allowing pressure out to the hose connector that also could be leaking. Turning the regulator all the way down might stop such a leak. If it stops yours then you know that's your problem.
The other likely place is the check valve between the pump outlet tube and the tank. I don't know if that compressor has a unloader valve or just a bleed hole in the outlet tube/line. The unloader valve/bleed hole releases the pressurized air in the outlet tube when the pump turns off. This allows the pump to restart with no air pressure inside the piston.
Looking on page 12
I'm guessing this uses the bleed hole setup. It's probably integrated into part #4. Run the compressor, wait for it to stop. You should feel some air drain out of that part for say 1 minute. If air continues to leak out after say 10 minutes then you know the check valve is not working. It could be dirty/something you could clean or you could just get another one.
One final place to check is the drain valve on the bottom. While it would be hard to check the regulator for leaks with soapy water, the check valve and drain valve would probably bubble if they are leaking.
