In any forward gear, the main forward drive clutch ("A" clutch) is made. In
"N" or "P" it isn't, however there is a known wear issue with two sintered
steel piston-ring type shaft seals in the back of the oil pump area that run
inside the torque convertor stator shaft. Oil pressure can bleed from the
pump past the rear seal causing a slight pressure to be applied to the "A"
clutch, causing it to slip but not fully engage. This wears it out
relatively quickly.
A quick test for wear in the piston ring seals is to ensure the 'box is at
normal operating temp, engage "N", and with the vehicle sitting on level
ground with all brakes off, raise the rpm slowly. If the vehicle starts to
creep forward at all before you get to roughly 3000rpm, then the seals are
worn. DO NOT hold the revs up waiting, you will cause wear!! The box will
continue to work like this for a long time, as long as you don't let it sit
in "N" or "P" for long periods with the engine running. There is a permanent
cure, but it involves a box stripdown..... replace seals with teflon ones
and do a modification to the "A" clutch drum and valve body.
BTW, no brake bands in a ZF, just multiplate clutches (7) and epicyclic
geartrains (3, from memory) and sprag clutches (2?)!
Badger.