According to TractorData.com the 5105 has a total hydraulic system capacity of only 8.5 gallons. So unless it was nearly empty, adding 6 gallons would put you way way over filled. Typically, if a hydraulic system is super over filled there is a vent tube somewhere that it will start blubbering out.
Any time you add fluid you should do so slowly and wait 5-mins for it to flow down to the sight glass. On my 2720 it takes so long that I typically have to wait over night to get an accurate reading. When you don't add red dye to the fluid it can be very difficult to see in the sight glass.
If you raise the 3PH and extend the boom and bucket cylinders on the loader it should consume enough fluid to cause it to drop below the add mark on the sight glass. Then as you retract the cylinders and lower the 3PH you should see fluid flowing back into the sight glass. Can you have someone watch the sight glass while you extend and retract the cylinders and 3PH?