OK. Your solution is very simple. I also assume that your lever was adjusted to engage as soon as possible.
Let me explain first what is happening as I had exactly the same problem after rebuilding the calipers. The seals simply pull the pistons back slightly when you release the lever. So, the next time you pull the lever the pistons need to travel an extra distance - hence the excessive lever travel. This same issue actually occurs when the pads wear off. So, here is a 10 min solution.
There is also a simple way to verify whether this is the case. Pump the lever vigorously about 5 - 10 times (squeeze as hard as you can and release as fast as you can) and on the last squeeze hold it and see if the lever stops noticeably further from the handlebar. If it does, that's the case. If you release, and try pulling the lever 10 - 20 sec later but only once, it will be the same old problem. All of these symptoms would confirm the issue described above.
Pull the pads from only one caliper. While the pads are out of the caliper, pull the lever ONLY ONCE and release. Try to fit the pads back in. If they go in easily and stay lose, pull them out again, and pull the lever ONLY ONCE and release. Put the pads back in. If they fit in snug - you are good with this caliper - keep the pads in. You will also notice that one pad seems to get tight first while the other one is still loose. So, leave the tight pad in place, and repeat the exercise with just another pad out. But only pull the lever half way at a time.
Once both pads are snuggly in, leave them in, and move on to the next caliper repeating all the same steps. You can try using partial lever pulls.
Once you are done with both calipers, don't be afraid to pump the lever and see how it feels.
Now if you accidentally over extend the pistons, don't worry. Gently push them in either with your bare hands (your fingers), or something flat (like a large flat screwdriver -just be gentle so you don't damage the pistons) - the key is to apply steady pressure for a few seconds to each piston which is out too far and try to apply pressure at slightly different angles - don't try to move a piston too quickly. Since you just rebuilt them, they will move with relative ease. Also, don't move them back in too far - all you need is to move them back in a fraction of a millimiter, so if you do need and attempt to push them back in, check frequently if the pad already fits in.
