Well, let's see.

The welding on the top nut did not go so well.
External Leaks!
RE-WELD!
External Leaks!
RE-WELD!
External Leaks!
Try brazing instead!

No external leaks at all, that is until it got to about 1800 psi, then POP the top blew off where the brazing was with a huge cloud of diesel fuel that sprayed everywhere!

There were no injuries but I did need to change clothes and take a shower!

But before it blew the top off, there were massive internal leaks that the pipe tape would not fix no matter how much I applied, and I even got some of that thicker tape that slowed it down some and the pressure would rise but it would not hold at all.
Looking at the design, those top threads were never meant to hold pressure, but the seal on the piston was.
Also my idea of a seal up top did not hold pressure either.

Also where the top nut contacts the barrel, there are no provisions in the original design to seal it.
If the fluid level in the external reservoir is higher than the top of the barrel, the air will vent through the tiny gap and the fluid will rise and then overflow leaking out, hence the original fill plug mounted only 1/2 way up the tube!
I solved this leakage issue with a tiny diameter "O" ring between the two.

Other than that one guy with the internal pressure leakage issue, I can't believe nobody else has mentioned, or had any of the other issues.

I'm going to duplicate the piston out of brass with a hole through it to the top for the fluid to pass through going directly to a "T" fitting that threads into it.

I will also have the original seal mounted lower in the tube than the pin hole so that it and the top nut threads will not have to hold any hydraulic pressure.

It will be just like the original bottle jack design only the piston will not move, and instead of the screw on top there will be a pipe thread fitting.

There will also be a Plexiglas spray shield between me and the unit at all times!