Automatic minecart dispenser with powered rails, detector rails and redstone, oh my!

Video here

You push the button, and a cart comes out. I'm having a few problems, though. For example, it's set up so that if too many carts get put in the hopper, it releases the next one. The problem is that if you press the button asking for a cart, the extra buffer space that is created is viewed as a cart and so the entire hopper cycles.

I was thinking of fixing this with some detector rails and a few and or xor gates or something, but, alas, I can't figure it out since when the carts are stopped in the hopper, they can't also be hitting a detector rail.

Any ideas?