Tilley lamp pulsating

Tilley 246B, I have replaced pip valve and immersed the lamp in water including the stop cock, neither the pump nor stop cock are leaking air (no air bubbles) yet lamp seems to be losing pressure and pulsating after a couple of minutes.
What to check next?

 

Are you giving it 70 - 80 pumps with the tank at least 3/4's full?

 

Probably around 40 pumps and maybe half full. I will try again with your recommendations and see how that affects it.

Thanks.