The serial number means that it is an old A series unit, made on 5/15/91, the 98th unit that day.
You have an old unit, many things could be starting to go bad. If there was sufficient caking in the head of the pump, then any loose dried oil can find it's way into the new check valve clogging it as well as making it leak. Even though you replaced the capacitors, did you check the centrifugal starter switch? If the contacts are burned or the spring bad, the motor will not start properly. Here is a link for troubleshooting:
mastertoolrepair.com/howto-clean-motor-s...-switch-p-23178.html

There is a very good chance that this is the case, but now you are getting into the finer parts of motor issues. I will need to refer you to a motor shop on this one! They may have a way of testing the rotor. (the spinning part)