Don't know if this applies to the TR6 pump, but it does to the TR3. The factory compression sleeves were a different design than modern ones, tapered on only one side instead of both sides. The result is that modern compression sleeves frequently will not seal, because the nut bottoms in the threads before compressing the sleeve far enough.
I solved my problem by machining a custom nut on the lathe, but
Scott Suhring's solution looks more practical.