I have never come across this but that don't stop me from having ideas.
If the spring is not attached to the pushrod then it is not doing anything. If the original pushrod had a method to anchor the spring, then it was a pedal return spring. As others have said, the dual M/C has the pedal return spring inside of it. As mentioned, the pushrod has to be the right length to park the pedal high-enough so that during use it will brake without hitting the floor.
However, the pedal also has to be up high enough that if a brake-line ruptures, on one end of the car or te other, that the safety system still works. And finally, it has to be up high enough that you can't put your toe under it and yank the pushrod out of the back of the M/C
But it cannot be so long as to prevent the compensating port from opening inside the M.C. When this happens, the pedal sinks lower and lower over time, eventually running out of brake travel inside the M/C..
Usually the pushrod is anchored inside the socket and it's a really good idea to have it working that way, cuz if it slips outta that socket it's bye-bye brakes.
I suspect the unusual thing you are feeling is probably the pushrod slipping back into the socket.
Don't forget, the large reservoir goes to the discs.
If the pedal forces it's way back up, then you probably have either boiling/contaminated fluid, in the calipers; or air in the system.
