60" Cub deck PTO problem

I need some help. I may need a lot of help, or maybe just a little. How much help is for our experts to determine.
What I'm trying to do is troubleshoot an intermittent uncommanded PTO disengagement.
Several suggestions have been tendered. One is to clean the air line to the bellows and make sure it isn't blocked. Another is to clean a screen on the bottom of the solenoid vacuum actuator valve.
Before I can really get into the troubleshooting, I need to get some facts.
First, I'd like to review my assumption of what components are in the PTO actuator system, and their functions.
1. The PTO clutch is actuated by a vacuum operated bellows assembly
2. The vacuum source of the PTO bellows is engine manifold
3. The solenoid going to the PTO bellows turns vacuum on and off as commanded by the dashboard PTO control switch.
4. The dashboard PTO control switch provides switched 12vdc to the bellows vacuum solenoid.
5. The vacuum solenoid directs manifold vacuum into the bellows chamber, when commanded. In this state, the PTO clutch is engaged. When the vacuum solenoid is unpowered, the vacuum is released, thereby allowing the PTO clutch to be disengaged. A release of bellows vacuum implies atmosphere is allowed into the bellows. Is this the case?
6. There are two small, flexible, black plastic lines running away from the bellows/solenoid assy. One goes to the carb/manifold area on the engine. The other disappears in a bundle that heads upward, out of sight, behind a side panel, somewhere under the dash. I don't understand this. If it were a simple atmosphere vent line, why wouldn't it terminate down by the bellows/solenoid assy? If it isn't a simple atmosphere vent line, what is it, and what does it do? I haven't been able to get enough access up under the dash to find out where this line terminates.
7. The vacuum solenoid actuator and bellows are an integrated unit. It appears easy to remove the solenoid actuator from the bellows. Two torx screws hold it to the bellows.
8 So far, I haven't seen anything in the way of a screen or filter to be cleaned, nor can I find the loose end of the line coming off the bellows vacuum actuator and running up under the dash.
As far as I can tell, the PTO isn't addressed in either the operator or tech manual. If I haven't missed something this blows big time.
My questions are:
A. Where does the black plastic line coming off the PTO vacuum actuator go up under the dash, and what does it do.
B. What should I be checking to find out why I'm getting an intermittent uncommanded PTO disengage?
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