We run about 16 quarter mile wheel lines in additon to pivots - are they mushroom or butterfly drains? If they are butterfly drains and won't seal, you don't have enough pressure from pump - try replacing them with mushroom drains - they don't take as much pressure to close.
Just in case you don't know what I'm talking about, I'll try to explain - a butterfly drain has two squarish flaps sticking up from the center "stem" that sticks through the pipe; the mushroom drains have a round concave rubber on top of the rubber stem that sticks through the pipe - both have the same style of metal clips on the stem end to keep them from popping through the hole into the pipe. you should be able to find pictures online somewhere that will be better than my explaination I'm sure. Anyhow, usually when the drains don't shut tight it's not enough water pressure - you should have about 60 pounds at the pump for a quarter mile of wheel line to get the drains to work and get proper breakup on your water.