Update on a really old thread. There was just no option for me to change anything on the PLC side as mentioned before but figured out a workaround.
The problem was that the pump could not be started if the stop bit held the 1 (or stopped if the start bit held the 1) You can visibly see a stuck button if the bit held the 1. The same button could be pressed again to attempt to write a 0 at release.
But instead of explaining that process, I am having the momentary of the start write a 0 to the stop and the stop will write a 0 to the start. This ensures the end user uses the buttons as intended without having to worry about stuck button or held bits. Seems to work fine.