I’m no expert by a long way, but simple physics tells me that when the fluid gets hot in operation, there’s going to be expansion. This is born-out by the dip-stick type level indicator integral with the cap, meaning the system is open vented (as are braking systems) via a breather hole in the cap.
Most old systems leak or weep to some extent, so like me, it’s vey tempting to over-fill the reservoir, to be on the safe side.
By over-filling, it’s likely that fluid will escape or spray out of the vent hole.
Or, it may be that the vent is blocked, causing excess pressure, hence leaks stemming from the crimping or other components not designed to be pressurized.
Just my thoughts, and willing to be corrected if wrong.