Rear Master Cylinder rebuild kit troubles/ losing pressure when holding pedal

Hello all! 
I've been slowly working on getting my 78' KZ1000D1 roadworthy. Everything is in good order, but for the last two weeks, the rear brake has been giving me endless issues. Master cylinder? Frozen. Caliper? Similarly frozen. Hose? Disconnected before the bike was parked, I wonder why... Latest tag on the plate says 1982, I feel like I should've seen this coming. Anyways, one rebuilt caliper, a new steel braided line, and a rebuilt master cylinder, and now here we are. Brakes come on hard, for about a second, then they die off. Take the master back apart? The main cup seal is sideways in the bore, jammed between the wall and the return spring. I reseat it (with the flat end touching the flat end of the spring), put everything back together, bleed it up; two pumps of the pedal and its back to 1 second of good pressure before its gone. Seal did the same thing again, jammed sideways.

Sorry, rambling rant over. Anyways;
Looking at seals online, I noticed something. I bought the $50~ master cyl rebuild kit from z1partsinc. The main cup seal in that kit has a little standoff as part of the rubber, placed on the inside center of the cup. Meanwhile, looking at the individual cup seal on z1enterprises, that cup has no standoff, its just a regular looking cup, at least with the pictures provided. Picture attached.  
The cup that I have currently installed has that standoff (unfortunately I do not have an image). Could it be that the piston is pressing against that standoff and applying off center pressure on such a small area that it tilts over sideways? Is the cup supposed to have that standoff at all? If anyone has any insight on this seal, please do chime in. That, or if you have any experience between either the z1enterprises or the z1parts rebuild kits. I really appreciate it, here's to a swift and straightforward process from now on... knock on wood.
Midway through writing, I realized I kept the old parts that came out of the master cylinder initially so I went out to compare. Lo and behold, the cups ARE different. The cup provided in z1parts' kit seems a little more shallow, and has a standoff in the middle, where the original Kawasaki seal sits flat on the piston. That kit claims to be a Koyama brand repro kit, so I guess something was overlooked on their part. Everything else in the kit is great, but that one seal is not correct, at least for my bike.

So, I guess Ill take this time to put out a word of warning. If you're rebuilding your rear master cylinder, double check that cup seal. That Koyama repro kit on z1parts seems great for a lot of its parts, but the cup seal is off entirely. Pick up a Kawasaki original cup seal from z1enterprises alongside the $50 kit, or spend $220 on the other rebuild kit on z1enterprises if you're feeling fancy. And be sure to unclog your return ports, that one evaded my mind for way too long aswell...
PS. what's that white cap looking thing in the Koyama kit? Top left of the image provided. No clue what it goes to.

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