If that is the issue, you can replace the seal but you’ll probably need a shop to do it. Shimano can send out replacement seals. They know what the problem is.
Here is short video showing the issue. Check under the brake hood and see if there is any oil there.
The only solution is to replace the seal or the lever (I did both and almost immediately blew the seals on the new lever….causing me to blow my own gasket. 
). It feels spongy because you are losing fluid. Holding the lever so the pads are against the rotor will only hasten the process as it will cause more fluid to leak.