I use 2 hose clamps with a short piece of flat aluminum between them to keep that rear collar tight. One clamp on the collar, the other on the shaft. The flat aluminum piece clamped between them keeps the collar from rotating. I believe Phak1 is correct about the cork packing wearing in. My collar continued loosing after a couple of tightens so I went to the clamp method and no more leaks from that rear seal.
