Auto top-up help

[Griffin]

I bought a kent float auto top up to use with my ro/di water except the pressure is pushing past the seal or the seal is no good (think the pressure though with the extra storage tank but no pump) so when i get home the sump is way past the shut off point = Does someone else sell or can i make a top off that will work the way i have it set up ie: water from ro/di system straight to the float. Or even a fix for the kent float. I do not have an area for a spare tank just for top up - I would even go for one of the auto top ups that are made to turn on a pump but instead open a gate valve - if someone knows of a electric gate valve that is salt safe.
Thanks for the look and info if you can share

Cheers
Rich