I have not personally experienced this issue. Your ZF 25 holds 0.75 liters of fluid. Is it possibly overfilled? You do not screw in the dipstick to check the level.
If not, it probably is a worn
sealing ring. If so, maybe this will help you:
5.2 Removing and disassembling the
actuating lever cover plate
5.2.1 Always set actuating lever to neutral
5.2.2 Remove hex nuts (22) from cover
plate (9), using 13 mm spanner (wrench),
and take off spring washers (4).
5.2.3 Remove cover plate assembly (12),
lever (6) and actuating cam (11).
5.2.4 Remove screw (7) from lever (6).
Pull off lever (6). Remove actuating cam
(11) and needle bearing (70) only in
models ZF 12 M - 15 M - 25 M - 30 M.
Remove actuating cam sealing ring (8).
6.3 Assembling the shifting fork in
gearbox section side shifting cover
6.3.1 Insert shifting fork (15) into gearbox
section (1) side the actuating lever
in such a way that the long arm of the
fork points downwards.
6.3.2 Insert shifting rod (16) through
bores in gearbox and shifting fork.
6.3.3 Fit screw plug (17) to gearbox,
making certain that the clearance between
shifting rod (16) and screw plug
(17) is min. 0.5 mm (0.02 in).
Seal screw plug with Loctite 242 (ZF 3
M - 5 M only O-Ring).
6.3.4 Check shifting fork for easy
6.4 Pre-assembling the actuating
lever cover plate
Use punch tool to press sealing ring (8)
into cover plate (9). Spread antifriction
bearing grease between sealing lips.
6.4.1 Insert actuating cam assembly (11)
into cover plate (9).
6.4.2 Fit actuating lever (6).
Clearance between actuating lever and
cover plate 0.5 mm (0.02 in).
6.4.3 Clamp actuating lever by means
of retaining screw (7), using a 13 mm
spanner (wrench). Screw in with torque
of 20 Nm.