Help with floor jack, leaked and won't pump?

Creedmoor said

Good info to know! Haven't had any jack issues yet, but I bet if I had, I woulda filled it all the way up and hydro-locked it.

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You can fill it all the way up if the jack is all the way down when you do it.
The thing to not do is to put more fluid in it that the return reservoir can hold is all.
Filling with it all the way down ensures you don't.