Is your system pressurized or a non-pressurized hydraulic system? As previously mentioned if non-pressurized the expansion tank is to allow for fluid heating
. Technically the proper tank size should allow for the full range of fluid expansion w/o overflowing.
Our vintage Wagner hydraulic system is non-pressurized and added an expansion tank to help monitor
. The expansion tank was also needed for the AP pump
. Added a self contained bleeding system so we could do it on the fly (if needed) and contain all the fluid. This really avoided the mess while bleeding and drawing air into the system if the reservoir got too low during bleeding.