With any double hose regulator (vintage or modern) the can should stay dry most of the times, but it is not uncommon to occasionally get some water in the can, and it is really not a big deal (I will explain further).
I agree with all the possible leak path that have been mentioned above, and you should try to find if any of those leak path is the culprit and try to resolve it. But, don’t be surprise if you never definitively find the cause of the leak. If for example, the leak was on the hose to horn connection, you may resolve the leak when you reattach the hose and you may never be able to confirm if that was it.
I will add that the most common leak path is just with the check valves in the mouthpiece. The check valves in the mouthpiece are just like the exhaust valves in a single hose regulator. They will in some circumstances allow a bit of water to go back even if they are in perfect working condition.
In a single hose regulator most of the time the valve is at the bottom and on every exhalation you blow any water out. But it is not uncommon for a SH regulator to breath a bit wet when you are in an upside-down position.
With a DH regulator the MP valves are normally vertical (in the normal swimming position). When air is flowing during inhalation the valve opens and if there is just a little bit of water at bottom of the mouthpiece that water can flow backwards, even if air is flowing into the divers mouth. I am just talking about a little bit of water at the bottom.
One of the big advantages of adding the DSV valve is that the diver (using good consistent DSV diving procedures) can avoid collecting any water at the bottom of the DSV mouth-piece.
With a vintage style mouthpiece, we usually try to blow it totally dry (every once in a while) by tipping our head to place the exhaust valve pointing down.
With the DSV, you can still do the same thing to blow out any water, but if you are always careful to close the DSV before removing it from your mouth and always blowing it dry before opening it, you can end up with a very dry loop even after an entire week of diving. I am actually experiencing dry-mouth for the first time in years of using a DH regulator.
Regular rinsing inside the can.
I personally only remove my hoses at the end of a weekend of diving or at the end of a dive trip. A dive trip to the Caribbean (or the Pacific) can be one or two weeks and can easily be 20 to 40 dives. It is not uncommon for the can to be dry, but (if I am not diligent with closing the DSV) it is also common to have about a teaspoon of water in the can by the end of the week.
At the end of a trip I always rinse inside the regulator can and flush water through the hose loop. I very rarely will remove the hoses from the mouthpiece. I dry the hoses by flowing lots of air through the hose loop. I have a dedicated vacuum cleaner that I just use to blow the hoses dry.
After the week of diving I normally open the can and rinse it well and either leave it open to dry or blow dry it with some compress air. I use an air blow gun that attaches to the LP inflator hose and sold by many dive manufacturers. I can dry all the crevices inside the can in seconds, so that I can close it and pack it to fly home.
I have also just run water into the horn and drained it out, but be careful. When you do that always make sure the diaphragm is at the bottom, so that the water is on top of the diaphragm. If you drain it with the diaphragm up, the water flowing down can (and will) cause a suction and pull the diaphragm, which will open the second stage.
By opening the can you avoid all that. And then by mounting in on a tank you can blow all the interior dry.
Saltwater in the can
Earlier I mentioned that salt water in the can is not a big deal.
On the other hand, salt water in the regulator inlet and into the first stage can be an issue, but even then our experience with the first stage in a Royal Aqua Master and the Conshelf has proven to be very rugged and be able to tolerate a lot of corrosion and abuse.
All the material in the regulator are corrosion resistant, but not corrosion-proof.
I have several Argonauts that I dive. Everyone of them were some kind of a machining error or cosmetic reject of some form. The internal plating in all of them is bad. Therefore, they tend to show a lot of green verdigris (copper-oxide) and inside the can they just don’t look good, but they work great.
There are very few spots where corrosion will affect any of the functionality and even then the very worst thing that could happen is the possibility of a leak if the corrosion is inside the first stage.
Inside the can, any corrosion will look bad, but it has to get really… really bad, before it would affect performance.
I am not trying to say that you should ignore it, but I personally don’t worry too much about it. I did not design the second stage (or any of the regulator parts) to be like a Swiss watch. I like to equate it more to an AK-47. It is designed to perform in a salt water environment without the need to treat it like delicate camera equipment or similar.
I inspect my regulators before and after every trip, but I have not rebuilt any of my Argonauts since I first put them together, about 4 or 5 years ago (or more).
Buceador con escafandra autónoma clásica.