[RCW]

I have never seen a vent on the top of the fuel pump on the 610, it is sealed and operates from crankcase pulses. If you trace the hose out of the curved fitting on top of the pump it will go through an oil separator and then down to the crankcase. If that system is not sealed at the pump you have a major defect in the pump.

Take a clean chunk of hose, connect it to the top curved fitting, and blow. If the air goes anywhere the pump is shot!

There is a sealed Crankcase Ventilation System that is not really serviceable, as there is nothing to break as long as the hoses still have their integrity. You can follow the small hose from the top of the inlet elbow on the air cleaner side of the carburetor air inlet and check that quite easily.

If someone ran the Mule with a cracked fuel pump diaphragm they may well have contaminated the oil enough to cause one or more of the compression rings to seize to the piston. That will then generate so much blow-by that it will pump oil to the point of not being usable. If that is the case, do a compression test.