The problem may be that the rear m/cyl piston is stuck with crud part way down the bore.  Get a new one off ebay - ZX 14 is the same. They are good quality despite the cheap price.  Remove the old one and open it up to find that it's full of crud which you don't want to pump into the ABS unit.
Use the original clevis yoke on the linkage and adjust it correctly before fitting the split pin.

Brake fluid should be changed every 2 years as it absorbs moisture and clogs up the brake system and the moisture and damge rubber parts. If it has not been changed since 2020 that is likely the problem. A thorough flush and new fluid is what I would try but you likely will need to replace the seals in master and slave cylindars. Good luck!

$400 for a f'ing fob??

$377.  But wow, that is a freaking lot.  Especially since you ALSO have to pay for the programming.  And I am not sure that even includes the metal key, which would also have to be cut (if one didn't have one already).

BTW- As I said before, you *must* have two working FOB's at all times to prevent possible disaster.  But even your dead active FOB will work as a spare passive one.  So you are still safe (assuming you have the other passive/thin FOB that came with the bike), just inconvenienced.

And this is also the reason my active fob NEVER leaves the inside zippered pocket of my riding jacket.  There is almost zero chance of losing my jacket, nor will the fob get crushed or wet.  I had a regular metal key for use with the bags and gas tank, I think it was $10?

Thanks, Maxtog.  I'll check with a couple of the local dealers and see how they react to programming a new one.

I had a similar issue with the fob a few years ago.  If I squeezed the fob it would work.  One day I got the NO TRANSPONDER message while riding, so I squeezed the fob in my pocket and message disappeared and all was well until I stopped about a half hour later.  When I turned the ign knob to off, it again showed NO TRANSPONDER for the 10 seconds until it powered off as normal.  No amount to squeezing the fob would wake the screen up after pressing the knob, so I used the passive function to start the bike and get home.

I subsequently found that the fob's circuit board was 'corroded' in one spot as shown:



I took it to an electronics tech who said he should be able to repair it by replacing that transistor, but he didn't have any that small on hand.  The smallest he had was 1.2mm leg spacing but this was 0.8mm.  He got 1 out of a $20 pack of 200 (minimum order) a couple of weeks later, soldered it in and the fob's been perfect ever since. 

Yours may be a different issue but look at it under magnification, or take it to a specialist tech to examine.