hot n dusty.
STOP REPLACING THE FILTERS!!!!!
There is no reason to replace filters or do anything unless the return filter pops as well. Ask Agusta for a letter (I got one) about the problem. Basically the problem is that the pressure switch is too sensitive and pops due to transient short duration pressure pulses across the filter, especially, but no only, when large demands are made on the hydraulic system (eg raising or lowering the undercarriage).
Agusta say that if the return filter has not popped, then contamination is probably not an issue. I have found that by leaving the system untouched, and just resetting the popout, the system will normally settle down after a couple of weeks.
One problem with changing filters all the time is that you can introduce air into the system, which will also cause the popout to pop.
If after leaving the system alone, you still have problems, then ask Agusta for the 1864-40 pressure switch (the standard one is a 1864-10). This is not so sensitive. You will get popouts for the first week or so while air works its way out of the system, but after that you should be fine.
We changed to the -40 and from the popout going every flight, we now don't have it popping ever.
The main thing though, if you have put new filters in, give it at least 2 weeks before you go changing them again. Give it time to work air out of the system.
There is no reason why you cannot fly with the popout popped. Agusta will say that as well if you ask them. If there is contamination in the system, the return filter will block. If there is contamination between the pump and the pressure filter and the pressure filter really does actually begin to block, the undercarriage will be slower to retract.
Once you get the system settled down (2 weeks without changing filters!!!!) any popout from then on should result in changing the filter, and then going back to letting it settle for 2 weeks.
Do not muck about with changing hydraulic pumps. There is no point. You will spend lots of money changing pumps, while introducing more air into the system making the filters pop even more.
There are possibly some aircraft out there that really do have some sort of problem, but in the last 5 years working on the 139, the above has fixed the popout problem on all the 139's I've had it occur on.
PM me if you need any further info.