The hold valve plunger is made with what looks like a martini glass on the end that goes inside the valve housing.  The "martini glass" holds the check ball and keeps it centered on its seat.  A common failure mode in the hold valve is for the plunger to break off at the "stem" of the "martini glass".  With the symptoms you describe, that is what I would guess is wrong with your hold valve.

  I had this same problem with my D14 when I bought it, and I ended up just going through the entire pump while I had it out and replaced all of the check balls and their springs.  Note that if there is a spring behind the check ball in your hold valve, it should be discarded.  There was a service bulletin on this, and it noted that the spring was often deformed and caused some hold valves to leak.  Leaving it out doesn't seem to make any difference in the operation of the system.  When you put it back together, be sure to replace the o-ring between the pump and the hold valve, as well as the one that seals the hydraulic tube to the hold valve.  Also be sure to re-adjust the traction booster linkage and the lift/lower linkage per the service manual.  If you ever get the chance to pull a plow or ANY Snap Coupler implement that engages with the ground, by all means do it.  The Traction Booster (draft control) system on these tractors is pure bliss!

Glad to hear that it was relatively simple to find the trouble.  I'm surprised you were able to buy the plunger so cheap.  I seem to remember it being more like 20 or 30 bucks?  I hope the bad cam follower didn't cause trouble with your camshaft (main drive shaft).    Regarding seat removal, I can't remember what size tap I used to thread it, but if you can find a drill that will fit the hole it will give you a good idea of what the I.D. is, then you can check a tap-drill chart to find the proper tap.  Somehow I'm thinking it may be 5/16-18?  After threading the seat, use a bolt with the same thread and some washers to pull out the seat.

  When reassembling the pump, I bought all new check balls, and bought a few extras of each size.  Some of the extras were brazed to the head of small socket (allen) head cap screws to make a device for grinding the seats.  To grind the seats, I chucked the shank of the cap screw in the drill press, applied a little bit of lapping compound to the ball and pressed it into the seat with the drill press running about 200 rpm.  Don't apply steady pressure, a few short busts of 3-4 turns should do the trick. I started with 320 grit and finished with some #2000 diamond lapping compound, but 600 will probably be fine.  Be sure to throroughly clean all of the grit out of the seats and pump when you have finished lapping!  The remaining extra check balls were used as sacrificial lambs.  As described in the manual, place a new ball on the seat and give it one good hard whack with a hammer and punch, then discard the now damaged ball and replace it with another new one from the same lot, then repeat this process for the remaining balls/seats.  This may seem wasteful, but check balls are cheap, and the dented ones make good slingshot ammo.  After rebuilding my pump using this method, my D14 will hold a 3-disk Snap-Coupler plow (pretty danged heavy) at full lift for weeks.  Sorry this got so long...

Andy:

Yes, the seat must be removed before the plunger can be replaced. The seat isn't necessarily destroyed by the tapping operation, especially if you lap the seat before reinstalling it.  If the seat is in good condition, the ball won't touch the edges of the tapped hole anyway.  Mine was pretty well beat, so I knew I would be replacing it and did not worry about the thread causing problems.  Regarding the check balls, yes you have it right.  A bronze drift might keep you from denting the balls, but I wasn't sure that it transferred enough force to be effective and didn't feel like experimenting.

Charlie:  It does indeed sound like you may have had a piece of dirt in the system somewhere.  I have noticed that when I leave the Traction Booster lever all the way up while pushing snow backward, sometimes the hydraulics will refuse to lift.  When this happens, moving the TB lever will fix the problem right away.  Perhaps I need to double check the TB adjustment when the weather gets decent.  Until then, I will have to remember to keep the TB lever down when I push backwards.