Also, Are you sure its an actual "hitachi" or a Hitachi "type" compressor. Dorman makes a crappy one that is a Hitachi "type". I have also seen, or better yet" heard" a fake AMK that was terrible. Sounded like crap.

Bearmach sells an AMK type compressor that is actually pretty good.

 

 

id suspect the compressor if there is an exhaust valve fault present.
but if you think you have an air leak you should really confirm.

step 1: park on flat area at off-road height overnight with battery disconnected-> this will stop the vehicle from lowering on its own.
step 2: record your levels
step 3: morning after-> check your levels and compare to what you wrote down.

if there is a huge difference at one corner -> check its feed line and air bag
if front or rear is dropped -> check that valve block.
if no drop-> you have no leak.but may need to repeat again set to normal height. bags can leak at different heights.also cold weather can effect them as well.

 

 

I was hammering on that. like any manufactured part, there can be failures.