Isn't there a spring retained from the original rearsets that keeps pressure on the back of the lever from where the plunger goes into the master cylinder?
If not, you may wish to get one and see if that solves your problem.
Also, anyone who needs the banjo bolt pressure switches, can get them here -
http://bellissimoto.com/Banjobolts.htmlUnlikely - the OEM rearset spring is round - goes around OEM rearset's lever pivot bolt and affixes to both the lever (on one end) and a notch in the backside of the rearset.
The Brembo plunger, however, does not attach to anything on the "piston" end. It can only "push" - it cannot pull the piston back - if the lever can move back up by hand and the rear brake is sticking - its unrelated to the plunger.
I've seen where people have removed the lever return springs because it moves the plunger's end away from the piston, so the first motion when pushing on the rear brake pedal would be take up the 'slack' or deadspace - you'd have to move the pedal down until the plunger gets to the piston before any brake action could occur. It was wasted movement.
The counter-worry would be that without a return spring, the pedal will continue to push on the piston and apply the rear brake. The real worry is not that the spring won't move it off the piston, but that the lever doesn't have enough upward free play to move upwards and get out of the way of the piston's additional retraction when the fluid gets hot and expands.