Tractor Repair: breaking 540 pto shaft on 1086 when using 499 haybine, internal splines, mechanic work

Question
I am a farmer in NE Kansas of 30+ years and do some of my own mechanic work (starters, water pumps, etc.) and all of my own servicing.
  
I have a IH 1086 with 4500 hrs. I broke the 540 pto shaft just inside the oil seal when using a NH 499 hydorswing haybine.  I removed the pto assembly and had the pto shaft replaced by a local ag mechanic with an after market part.  After using the repaired pto less than an hour it broke again in approximately the same place.  This time it appeared the the replacement part may have been flawed because of the dark color in the center of the break.  I again removed the assembly and took it back the the local mechanic.  He replaced it with another after market shaft from a different supplier at no charge.  While he was working on it the second time I used my IH 806 for a couple hours to finish windrowing the field with no problems.  I even plugged it once and could hear the oil bypassing.  I hooked the 1086 back to the haybine when I started another field and broke the pto again in less than an hour.

Am I just having bad luck with after market parts or is their something possibly wrong with the haybine hydraulic pump?  Your thoughts and comments would be greatly appreciated.  

Answer
Obviously there is nothing wrong with the haybine if you used the 806 on it with no problems, and if there was something wrong with the hydraulic pump on the haybine, it probably wouldn't cut hay.  Normally, output shaft breakage is from turning too short or not having the drawbar set to the correct length causing the implement shaft to put lateral force on the pto shaft.  But if I remember correctly, the NH 499 has no drive shafts and is driven by a pump that mounts directly onto the pto shaft of the tractor, so that eliminates those possibilities.  I know these pumps are quite heavy and can put alot of stress and wear on the pto shaft, but I have not seen them breaking the shaft.  Inspect the internal splines of the hydraulic pump for wear that could cause the pump to wobble or chatter during operation.  Grease should be applied to the pto shaft before installing the pump to prevent wear.  I have seen the splines on these pumps strip completely out.  Check the drawbar and the torque arm of the pump that attaches to it to keep the pump in position.  Is there a drawbar extension that must be used with this machine?  Is the drawbar set at the correct length? Is it possible that the pump is striking something when turning?  I am not sure about the use of aftermarket parts.  I have not heard of any problems with them so far.