For k_p
It is highly recommended that, if you are going to take apart the pump, that you replace all the o-rings in it. O-rings do not last forever and are quite cheap, so there is no sense in reusing them. The only thing that can come out from this is having to take apart the pump again at a later time to change those o-rings when they start leaking.
The red jack in the thread do use metric o-rings, but the o-rings for the big nut and the pump piston can be substituted for SAE o-rings which have similar sizes. I mention this because many local hydraulic repair shops do not carry an inventory of metric o-rings because these are more expensive than SAE o-rings and the owners think that they will never sell many of the metric sizes. So the best they can do is exchange the metrics for SAE sizes whenever possible.
You can always order the correct metric size from The O-ring Store, if you are going to order the u-cup. All you need is a caliper to measure the thickness (cross section) of the o-ring and the inside diameter of the o-ring. But this I D is measured on the metal part where the o-ring acts. For example, to find the ID of the o-ring in the big nut, measure the ram's diameter. Do not measure the o-ring thickness in the worn part of the o-ring, which is the inside of the o-ring.
There is also an o-ring inside the release assembly. Just remove the nut, the gear and push the gear shaft so that it comes out the other side and replace the o-ring. Other similar jacks, like the one shown in TxJohn post, have a welded gear so the whole release assembly has to be replaced when the o-ring in it leaks.