I was working with minecart-magma elevator and I have made a mistake by making TOP of magma sea a BOTTOM of my elevator and, as it looks that magma sea doesn't want to refill I might have found out what is minimal amount of magma needed to fill a cart. When magma level is at 5/7 it won't fill at all but when it is 6/7 it will drain square donw to 4/7. I have non-bumping chain of around 21 carts and as magma level drops I'm having more and more randomly empty returns (and I think that randomness is tied with fluctuating level of magma surface).
It seems that after the latest patch (or somesuch, been a while since I patched), magma now has pressure, like water. In other words, in an U-pipe, it'll rise to the same level on both sides. Can anyone confirm this? I just had Fun happen thanks to an overflowing lava moat. :d