This has 99.997472% chance of being a waste of your time.

A ballon of Helium floats, because it is less dense than air. Grab a paper and pencil.

First, there is rubber ballon and a mass, tied by a rope. These are dimensioned so that the forces balance, and the hole thing stays at same height. No problem there. 

Then you get a couple of hoses, and two pumps. These pumps have two entries, one out and one in. You cut two holes on both sides of the ballon, to the right and left of the vertical rope, and glue two separate hoses on these holes. Their extremities are dangling. 

The left hose coming from the ballon is fixed on the the output of one of the pumps, that is placed to left and a little under the ballon. 

The hose of the right you fix on the input of the right pump, on the right on the same level as the first pump. Then you get a third hose, and connect the out of the right pump to the in of the left pump. 

This forms a closed loop, and now no Helium can escape, it will just fill or empty the balloon. If you energize the left pump, the ballon fills up, and the mass lifts. If you energize the right pump, the ballon empties and the mass falls down. If you alternate the pumps, the ballon-mass oscillates, albeit slowly, up and down.

Now here is the trick: You would attach a point of the rope to a linear to circular motion system. Then you take the output of the rotating shaft and connect to a generator.

If you made it through here you are a champion.

Would this whole contraption, produce more or less energy than required by the pumps? 

No matter how many times Physics lectures you attended, sometimes you just can't resist the urge to develop a free energy machine.