May or may not be pure isobutane. But that does make the propane/butane graph shown above irrelevant.
There is one for isobutane posted around here somewhere.(Isobutane vs Nbutane - #9 by cyclopath)
Re-reading your initial post, I see you’ve already done the initial volumetrics, and know you’ve got 16liters of solvent in there (good on ya!).
Now we need to figure out if there is room to put all that solvent elsewhere in your system so you can repair your solvent tank and get that prv installed
Filling with water is a solid solution. I generally use a tape measure and ask the googles to do the maths.
In your case (metric) it’s easier…cm3 being equivalent to mls and all.