There is a spring on the back side of the toilet that could be weak as you said or there could be other binding of the mechanism.. The only way to replace it is to take the toilet off the floor, replace the spring and flange seal, then bolt it back down. Ours is doing the same thing but I can lift the pedal with the top of my foot after flushing and the seal holds water. When we get back home from our current trip, I will replace the spring and flange seal, less than a half-hour job. The spring and flange seal are available from
Amazon as a kit.