Ok so I looked it up and it is very interesting. The gun's steam emissions seem to have been a problem because they gave away the gun's position. Thus vikers designed a number of solutions. The vickers gun is a very similar gun to the maxim and may look similar at a glance. First a steam pipe was made that was dug into the ground and used the earth to cool the steam, but holes had to be redug from time to time. Then a water bag was issued which cooled the steam so it dispersed and did not give away the gun's position from as far away.

Then, what you are thinking of, a condenser system was developed especially needed for recycling of water in arid environments. The hose from the steam vent was kept, but redirected to the condenser can and a hose from the condenser was fed to the water filling port. This condenser system used the steam generated by firing the gun to drive the system there were no pumps. The steam pressure forced water from the condenser can up into the water jacket and then condensed into the can's water reservoir to wait its turn to be fed into the gun.

All these systems were optional and screwed on inplace of caps on the water filler port or steam vent, so you will see the same guns with them or without them.