D17 IV Lift Arms Won't Lower

I know this is a common problem, and have searched and read the various posts regarding corrosion and hanging up of the Traction Booster and Position Control Linkages.  Everything is free, and with both levers forward, there is free play between the end of the piston coming out of the Traction Control and the metal tab connected to the linkages that pushes it in. 

However, I can't get the lift arms to lower except if I use a 10 foot board as leverage to force them to lower.  A 3 bottom plow on the hitch will lower it straight off, so it is not that it won't lower at all, but it requires excessive force. 

Is it possible that the "button" coming out of the Traction Control hydraulic body itself is sticking, and not releasing pressure?  I opened up the "rate of lower" little screw on the bottom of the valve to 2 turns, with no change. 

Again, the linkages seem to be free, and there is free play between the piston coming out of the body and the steel tab that pushes it in, so it doesn't seem like it should be anything binding in the various linkages. 

Just for background, the tractor has been used only intermittently by the PO, and has sat outside, but it not in terrible condition.  I went through everything with fresh fluids everywhere, and it has been running and working well.  It seems this hydraulic lowering problem is more recent, as it seemed to work ok a few weeks ago.

One thing I noticed is if I pull the Traction Control all the way back with a plow on it, with the main lever in detent, the plow will raise, then the pressure gauge will max out, and I hear the relief valve.  If I put the Traction Control forward, then move the Position Control full back, the pressure will go to about half max, like the manual says it should. 

Any ideas?

Dr. Allis,

Thanks -  was hoping you would reply.  Actually, I converted it over to factory 3 ph from a 185 salvage.  I have the link rod from the 3 ph tortion bar connected to the Traction Control, but the belcrank has some free play, and there is also free play between the piston coming out of the Traction Control module and the steel driving piece coming off of the Traction/Position control linkages.   I can jump up and down on the lower hitch arms, while pushing the lift lever all the way forward (or wiring it forward), and can't get the arms to lower.  If I wire the lift lever full forward, and use a long 2X6, I can force the arms down with considerable effort.  It doesn't seem to matter if the tractor is running or not.