I replaced the hard rubber gasket with a soft silicone one and gave it a good coating of silicone grease,
added a paper washer and silicone grease to the front threaded rod that holds the sub together
added a little grease to the grease port and sealed it with some tied off tube.
Again I added 3psi and looked for bubbles. The main seal was totally airtight, but there was still large bubbles coming from the front, so I added another 2 greased washers and added 3psi again. This time the front was airtight and the only bubbles I got were some tiny bubbles from where the main shaft penetrates the rear WTC bulkhead and some bubbles out the end of the shaft. I'm going to apply some epoxy around the propshaft to seal it to the WTC and once the shaft seal is fitted it should be totally air tight.