FG 84 getting stuck in reverse

[Larrylandcruiser]

Hi all hoping to find some help/ideas here as the Fuso dealer option are running out of ideas & the costs keep going up.
The truck 2011 fg84
Problem: when i put it in reverse after starting up it literally shoots backwards 5ft even with foot still on clutch.
So far had new clutch,pressure plate and spigot bearing replaced (not as a solution to this problem but needed new clutch)
Problem occurred with old clutch
Tried new master cylinder- same problem.
Fuso now wants to try slave cylinder but no guarantees and at $800 ++ it is costly option- for those that dont know you have to remove g/box as internal slave cylinder.
Only ever happens when i select reverse.
Truck does sit in one place for a few weeks at a time - thoughts are maybe the inout shafts are getting rusty as very fine toletances.
BUT does not happen in fwd gears.

Is it a problem with the gearbox?

Any ideas greatly appreciated.
Going to drive it every day for a few weeks to see if that fixes limited use issues

 

[gait]

I really have no idea but maybe look at engine / gearbox mountings?

 

[kerry]

Does it grind going in to reverse or are you starting the truck with it in reverse? If it doesn't grind, that implies the clutch is releasing but if it is, how can the truck move? Have you tried pumping the clutch multiple times before selecting reverse to see if it makes a difference? After the truck sits, have you tried using a forward gear before selecting reverse? Does this stop the problem? Does it only happen immediately upon using reverse after sitting and not again until it sits again? Is there an access hole to stick an inspection camera inside the bellhousing to observe what is happening with the slave cylinder?

 

[Larrylandcruiser]

Txs Gait & Kerry
Doesn't grind going in reverse. Tried pumping clutch-seems ok. Treid fwd gear last time, no issue going fwd so thought no issue going in reverse- but i was wrong.
Yeah only happens after sitting and then not again till it sits again. There is an access hole so good idead with the camera but obviously not safe to check the actual problem.
Cheers

 

[engineer]

this happened to me yesterday on the island!! i think it may have something to do with HSA being a gremlin.
I'll have an ask around today and ket you know

 

[Larrylandcruiser]

That would be great, thanks.
I notice you have updated your location from Cairns