I'm going to make a few leaps here without looking at the contract... (dangerous, I know), but I believe there is a restriction in PBS that prevents it from scheduling you for more than 18 days on duty. When you tell the system that you are on mil leave for 20 days, it treats each of those days as a day on duty, therefore it can't give you any more duty days. Of course, I could be wrong...

KC- I think your best course of action if you want a full line in addition to your mil time is to wait to drop your mil until after the schedules come out. I know, they want you to do it earlier, but they can't force you to do that. In fact, there really is no such thing as a request for military leave. By the law, all you have to do is tell them when you're going to take your leave. There is no reason at all to take mil leave on a scheduled day off. You are not abusing military leave if you do it this way.

Look at it from another angle as well...let's say you had six days of hard mil duty for something like a mobility exercise or some other such exhausting crap that nobody likes to do. Would you really want Delta to make you fly a full 72 hour line in addition to that? Reducing your required line value for military leave is a good deal in the long run. Might not seem like it when or if you are struggling to make ends meet on 3rd or 4th year pay.