Right , well I was suffering the same with having to pull very hard and letting the hose bounce back up in order to shut the Univalve, then it came to light that the spring was rusted and snapped .
Exceed sent me the new spring and it was cured , however since then it has gone back to the way it was , not as bad but not quite right.
I have 2 Univalves , 1 original with full stainless spring and 1 new model with the revamped thinner galvanised and stainless mix spring , the older model is far superior in my opinion.
I do feel that the newer spring does not have the rebound that the old one did .
Lee GLS got some full stainless ones on eBay that had pretty much the same dimensions , he had to chop a coil off to fit it , but I believe it worked well .
It would definitely be worth getting a bag and doing a bit of trial and error as once someone gets the correct amount of spring to cut off and shares the info then we can all revert back to full stainless springs .