Well you should definately consider a fluid change and full brake bleeding.
I suspect you may have air in the system.
In any vehicle, if I ever had spongy brakes, it was due to air in the system.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.