Navtec IA6-10 Backstay fluid top up

[Wainui]

I have an old Navtec backstay adjuster (Round cylinder square bottom) that leaked some fluid when the release valve was mistakenly undone all the way. How does one top up the fluid on these older unit? I can't find a etra port, only where the release valve is. The unit works well but the guage is unreadable.
Many thanks