You can't "overfill" that thing, only underfill.
Let's say you fill it to the top of that fill tube. When you're running the tractor, all that could happen is that the fluid will pop off the plug and spill out some, no biggie.
gc2400 gallon overfull fluid foaming and I could not move in high range.
dropped it back to 1/2 gallon over and could move when hot.
heated it it all up drove to neighbor with garage with level pad, drained another .75 qts out.
tractor fine for well over 200 hrs.
yeah I am a bit behind on hydro on my OWN GC tractor while the other GC/MF tractors around me are good.
BUT if you say you cannot overfill a GC I will call you out.
first GC I dealt with was a 2300 in 2004.
in november 2004 the GC2300 was a new item to dealers.
dealer over filled. by 1.3 gallons
one friend bought one and was pissed.
new release to dealers so I dealt with massey themselves.
customer (my friend) pissed off.
I drained a gallon out (so still .3 overfull) and tractor worked fine for may years. over many owners. all that knew me.in 2010 this tractor had 845 hrs on it.
I bought a GC series in Nov 2020 but I've been dealing with GC series since 2004.
anywhoo......I've been involved with GC series since April 2004. yup that date sounds odd. cause the actual release was many months later then the test release.
so....long story short....YES YOU CAN OVERFILL A HYDRO TRANSMISSION SETUP.
ANYONE that tells you otherwise is mistaken.
EDIT: in 1984 manual (bolens) specs wrong and overfilled a 14hp Bolens by 1.1 gallons.
when cold everything worked.
when hot you were screwed, mower deck would not lift, tiller would not lift, snow blower would not lift, mower deck would not work at all.
all these things were NON pto and hydro driven, and when hot would not work.
spent many hours on phone with bolens, identified the misprint in service manual, corrected problem and my grandfather tractor started working,
1984....dam I feel old.
was....eerie...dealing with bolens engineers while I was only 16 at that time. whatever.
Had similar calls that year with Case (before fully swallowed by Ingersoll) on their garden tractors.
damn I feel old.