Well, I've been working on the USD Aquamaster for many years. I was trained on it in the US Navy School for Underwater Swimmers in Key West, Florida. There are some very good suggestions above, but there are a few things that were not said about the Aquamaster.
First though, you can get a complete description of how to service the Aquamaster in the US Navy Diving Manual, March 1970. It goes into great detail about how to service the Aquamaster, including the settings mentioned above about setting the interstage pressure at 110 psi. However, the US Navy used the old Aluminum cylinders (which could not be hydroed, by the way) which were 3000 psi. So they set the interstage pressure to 110 psi on a full cylinder, at 3000 psi.
Second, do not attempt to replace the LP seat holder unless you actually have a leaking LP seat. Most of the original LP seat holders used a nut and washer that was not a locking nut, and so they were either filed or punched with a center punch to preclude the nut from backing out. There is a note in the manual stating, "Be sure to use litharge cement and counter-punch the nut (25) and the disk shaft (33)..." Therefore, you may not be able to remove it without cutting it off (which was the normal procedure. So, before you try to remove the LP seat holder, be sure you have the correct replacement if this has been done to the LP seat holder.
The Vintage Scuba Supply link below is good, and has new hoses, mouthpieces and valves. It is quite possible that you will need new mouthpiece valves, and a new flutter valve. The old one may be rotted into the metal of the bottom box. If that's the case, you will need to remove it, and the best way I have found is with a thin blade of a pocket knife. No solution will touch it, so don't try.
When you put the mouthpiece/hose assembly back together, you must balance it so that it can stay in your mouth without pressure. There are two ways to do this. Either place the regulator down on a table, and aim the mouthpiece straight back at the regulator box, or hold the mouthpiece balanced on your fingertip about three inches above the regulator and in front of it. If it balances there, it will be balanced when you have it in your mouth.
A double-hose regulator should sit low, between you shoulder blades, when worn diving. This position is easiest to achieve by using double-tanks (see the photo, enclosed).
Finally, if you see any of the tanks in the enclosed photo, they must be drilled. They are 3000 psi aluminum with a plug in the bottom, and used a very old technology from the 1960s to give the Navy UDT/Seal teams an aluminum tank set (anti-magnetic). They were never made to be hydroed, and some have found their way into the civilian market (at least, I saw some in a school welding shop in the 1980s).
I hope this helps.
SeaRat
