I had 3 belts go in as many months and they wee all fitted to spec and rechecked after i bit of umming and arring it was decided that the crankshaft pulley was to fault as it had a deep score in it and was sucking water in when hot and getting dunked into cold water as GBMUD and simonr has said above the thing to do is to fit a breather to the case just like an axle breather that is one of my next jobs but for now i have just stayed out the wet stuff and the belt has been fine even after tweaking the pump which would put the belt under more strain and all is well hope yours is the same thing as it is an easy fix but it is frustrating the first couple of times
Frank