I have a similar problem: my right arm makes excellent lead position, can stay extended for a long time, but my left arm would drop quickly. HOWEVER, even though I breath to the right, this problem doesn't seem to be caused by breathing (unless someone could figure out that it actually is caused by it). Here is why I say it's not caused by breathing:
1) Even when I do not breath, the problem is still there.
2) When I breath to the LEFT, the problem is still there: it's still the LEFT arm that drops when extending, and the right arm still makes good lead arm.
Basically, during a length, I gets more propulsions when rolling on my right side. My right side seems to be a much better swimmer than my left side!
Perhaps no one can tell me what's wrong with me, but if someone can even make a guess, please let me know!