mac,hey Chris , I've been through most of what your talking with J30 p/st. Where is it leaking ? , around the back of case there's a 3 1/2" "case" O-ring ; front shaft seal leaking ? And (rarely) I believe an internal seal ,that would reduce available output pressure. Yes I would keep the OEM one from the car , because I had only the large case O-ring leaking .I found a kit and beware some may not have that large case O-ring,beware to check for it - that's one of THE leak points.Kit was $16 17 years ago. I would go ahead with buying -"what your talking about" -Tip: good mech. friend (and me) says cardone [email protected]
Now IF & when you do the rebuild you gotta be super meticulous, especially with about 30 little 3/8" square "impellers" which"ride" in groves in a round drum which forms the pump's action , just me but I PUT EVERY PART,no matter how small , plain-looking ,whatever IN EXACTLY THE SAME ORIENTATION HOW IT CAME !!!!!The little squares for example have an "orientation" - a side touching back, a side touching the "side-wall" , a side & DIRECTION (especially) with WEAR on it's edge that MUST be positioned SAME back together to prevent chewing up / self destruction! You can temporarily stick them in a "stretch" of "Silly-Putty" stood in a row!
Lastly , this was done @ 110kmi (now @ 260k) and after I would flush every 4 years with Mob1 ATF , so far 4 flushes and the pump and ESPECIALLY my rack have'nt so much as a drop of leakage with frequent enough fresh - quality fluid !
p.s. Do keep the old one, many good reasons , even if core is over $50 !!