I am replacing a 20 year old sink pump in my unfinished basement. The pump pumps the water up about 6 feet where it joins the discharge from the rest of the house floor. The old sink pump had a copper pipe the went from the box and curved back into the sink, this allowed air to vent the pump box. The new sink pump has a place where a 1.5" vent pipe attaches and I'm not sure what to do with this. Can I just leave it empty? Or am I suppose to attach it to the house venting pipes that go to the roof? None of those are anywhere near the sink or even in the basement. What purpose would it serve? There is a check value on the drainage line so water can not back into pump.

The old sink pump had a copper pipe the went from the box and curved back into the sink, this allowed air to vent the pump box
The problem with that set up is that the pit vent not only draws air in as it discharges it also expels gas out as the fixture drains into it, If you don't pipe it outside or revent back into the house system you'll be faced with the same problem. You'll be discharging pit gas back into your home. Solution? Vent the pit anywhere but back in your home. Good luck, Tom