You think its leaking because the main cup isnt parallel to the crank axis to the extent that the gland cant compensate?
that sounds a bit scary!
obviously there are quite a few repercussions to misalignment down there which can cause tears, but realignment of welded cases, is a tricky and costly business…
yes .. A seal would .. well .. seal.. and is quite a simple procedure although there are a few ways of doing it in circulation.
Some people make a bronze top hat bush that is pushed into the rear aperture of the mains cup and that carries a seal in the middle. The ‘brim’ of the bush serves as the thrust face to the original roller plate.
Ive seen cranks with hardened sleeves fitted to run seals on but you can also run them straight on the crank which is simpler and this is what we do..
We make Alu bronze internally screwcut roller plates to the appropriate sizes to run directly against the cup gland face and push a seal straight into the rear aperture of the cup..Simple.
As Erik says.. you need to use non return valves. We make them to a design my Dad hashed out years ago using small discs as the valve but there are other methods using balls… and i am sure you can get something like this commercially ..