Armitage Shanks dual syphon/rim flush toilet repair ?

I needed to replace the diaphragm in an old toilet. I decided to fit a new 2 part syphon to make the job easier/quicker next time. When I removed the old one I found a peculiar brown tube inside the old syphon. Having put in the new syphon it did not work well. So I installed the brown tube in the new syphon. This helped, but is still not as good as before. I have also found a black seal which must go on the small 1/2 inch part of this tube. WHERE and which way up, at the top of the tube or flush with the ceramic ???