My Iwata doesn't hold the pressure any longer

I wouldn't leave it on all the time unless it was frequently being used during the day. Kinda like a compressor in a tire or woodworking shop where its used off and on during the day. And I think most of those kinda places turn them off after working hours. To easy to have a small leak when no one is around and have the compressor running through the night.

Cadet Chuck

turn it off when not in use.

Maybe it has something to do with the fact that you are leaving it turned on all night, and presumably all day, too.  This could contribute to stressing seals and other components to the point that they are wearing out and developing a slight leak.  I would suggest you turn it off when not in use.

If you are certain that you don't have any external leaks I would suspect a loosening cylinder head or dirty non-return valve.

You can check the cylinder head plate easily - get a hex key & ensure that the four bolts that you can see on the cylinder head are reasonably tight (no excessive tightening required).

The non-return valve is fitted on the top of the cylinder plate, which is the directly below the cylinder head, the valve can become contaminated with gunk (a combination of dirt that has passed the intake air filter & shed main seal material (nothing to worry about)) & fail to seat properly - the valve can be striped & cleaned.

If you have a look at the Iwata parts diagram, the non return valve is on page 3 & is the Check Valve on the call out to the left;

http://www.iwata-medea.com/images/iwata-pdf/Smart%20Jet%20OM_REV2.pdf