[audietron]

A way to see why it is not disengaging is to take the shifter apart and pull the coupler all the way back and look inside the shaft to see where the flat part of the plunger is located. It is possible to pull too much also if you are using a spool to wind up cord make sure it's a small diameter to put less torque on the balls. If you indeed need a bigger bore cylinder here is one from mcmaster : it requires making a custom bearing block but it should work with a little over 2 times the force for disengaging.

http://www.mcmaster.com/#6113k15/=qqez9a

Good luck!