DON'T cap it. It's the bowl overflow if its small but if its the larger one like in the pic below, its the weep chamber drain end and it too has to be open. Better to run it without it then cap them...either one. Just stay out of deep water.
Not a check valve but it works a little like one. It lets any coolant or oil that leaks into the weep chamber drain out but makes it hard for trash and water to make it back to the chamber.
It shouldn't ever leak unless the mechanical seal...or the oil seal.. is failing..and by it leaking it is so that will need replaced. If you plug it the pressure will force coolant back through the oil seal on the other side of the chamber and into the crankcase so..NO plugging, get it fixed.
Consult your service manual but I think so...and you will want to also replace the oil seal. If the shaft is warn you will also want to replace it as well.
