Tractor Repair: General Tractor Hydraulics Question, tractor hydraulics, shear pin

Question
Hi.

Here's a (likely) simple question on tractor hydraulics.

Background:  a while back I had a PTO pump and hydraulic fluid reservoir added to my IH 624 tractor (German built).  The original Bosch hydraulic pump was all but shot (still runs the power steering, but that's all), and I couldn't find a replacement.

This past weekend, the hydraulics seemed underpowered. This has happened before when the fluid level dropped (there is a leak in the system so I add fluid every once in a while), and each time I added more fluid the hydraulic worked better (likely due to better pressure?).  This happened again over the weekend, so I added more fluid.  The fellow who installed the pump and reservoir told me to keep the reservoir level low (under half - I assume for pressure reasons as well), and I've always done that.

This time though, after starting the tractor back up, a clean hole was blown through the bolt that plugs the filling hole, causing fluid to spew out (the hydraulics still work when this is happening, but fluid flies all over).  I'm thinking I may have over filled the reservoir, causing the hole.  Now, this hole in the bolt looks perfect, almost like it was designed to be a 'safety check' on excessive pressure, to save the pump from damage.  I've never heard of, or seen, a bolt like that.

I need to replace this bolt before I can use the tractor again, so I'm trying to figure out if this is in fact some kind of special bolt designed to be a weak point in the system (best analogy I can come up with is a shear pin for a snow blower) so that I can get a replacement.  If there is no such 'special bolt', I'll just get a replacement and make sure I keep the fluid levels even lower in the future.

Your advice is greatly appreciated!

Thanks,
Corey

Answer
Hello,

  This is an unusual question because you are not using the original hydraulic system and have added a pto pump and reservoir.  I have never heard of anyone doing this instead of replacing the original pump, but here are some thoughts.
  There should never be pressure in the reservoir, it is on the suction side of the pump and is only a place to store oil for the pump to use.  The reservoir should not be filled too full because the level will fluctuate as the hydraulics are used.  For example, a hydraulic cylinder holds more oil when it is extended than when it is retracted, so the oil level will be higher when the cylinder is retracted.  Also, you need to have room for expansion when the oil gets hot.  There needs to be a breather or vent on the reservoir to let air in and out as the oil level changes.  The bolt with the hole in it that is in the fill opening is a vented plug, so it is supposed to have a hole in it.  It's possible that the vent hole was plugged with debris and finally blew out when you overfilled it.  That would also explain why the hydraulics worked better after you added oil.  If it was not venting properly, the reservoir could have gotten a slight vacuum in it when the oil level dropped and the pump was possibly starving for oil until you removed the filler plug and let air in.  I don't think you need to replace anything.  Try running it again with the reservoir only half full and see how it works, and fix the oil leak if possible.