I  think I asked this  a few years ago,  but cant  remember---  

on my  3680,   Massey  went with a  hydraulic  servo  to  shift  from   rabbit  to   turtle.      Previous  generations  used a linkage.

today,  I  couldn't get it to shift  from  high range,  to low.   I made sure  the  4x4  wasn't engaged,  foot wasn't on the brake  (which engages  4x4)... still  wouldn't  shift.  
I'm  aware the switch  in the center of the  round shift cover is supposed to interupt 4x4  as it  shifts, thus relieving  gear bind for ease of  shifting... but  made sure it wasn't in 4x4  just in case that  switch is  not  working.

I  disconnected the wire to that  solenoid--  no change.  

One  time,  I was able to  rock the tractor  back and forth a little  and it did go into low.  

I believe how it works,  is  when asked to shift into high,   hydraulic pressure  pushes the piston,  which is connected to the  shift rail on the hi/low  range.
when  asked for low range,  pressure is bled off,  and  (I'm assuming)  a  spring  return  pulls the  shift rail into 'low'.   The servo is  one-way only. 
I've looked, and looked  in  PARTS,  but can't seem to find  any sort of  spring,  or mechanism to  pull the  shift rail into low range.    
please advise if  there is some other  mechanism that  does  the  work   for pulling it into low.   It  about HAS to be some sort of  spring doesn't it?


I'm going to do  a couple things tomorrow-
1.  I'm going to disconnect the  supply line and cap it,  so  zero  flow gets to the  servo.  

if it  shifts  back into low at this time

2.   I'll pull the  spool out and  check it for binding.      I replaced it this summer,  as it was bent up  from hitting  a hidden limb in the grass I didn't see.    Found a  NOS to replace it. 

Back in July,    the right side  hydraulic pump ate itself,   and  was replaced.     there's a chance  a  bit of  shrapnel finally  made it  to this  spool,  and  is not allowing it to close--  which would be allowing  pressure to reach the shift piston.    

I got some  investigating  to do yet.