If all the buttons work but those 2, then you need to repair those 2 buttons. Review the button size against the sizes available at this site:
[Link: buttonworx.com]

and get the one that has the right size for the repair.  Unfortunately they do not have a complete set for the MX-500.  They do have ones for the MX700 - MX850 but they will not work with the MX-500 because the buttons are actually in a different position.  You'll need to get individual "buttons" and repair yours as needed.

I've used the MX850 ones and these things DO work.

BTW, heed the comments in the video and do not push down on the button fix when you install it. If necessary, use a small object (i.e. very small screwdriver) to press lightly on the corners of the tab once applied. You just want it stuck until you get everything back together as it will set itself then. If you press across the entire pad, you could deform the pad fix and the button may act as if the button is continuously pressed.