So what's the deal with Ford changing the oil capacity on T9s? Some were 1.9 litres, and others 1.4 litres, but was there any change to the gearbox itself apart from the height of the filler hole? I ran a 1.4 litre box with 1.9 litres for a few days before draining the excess, and swear it shifted more smoothly, so I'm wondering whether there's anything inherently wrong with bumping the volume back up.
 
Thanks.

Wasn't there a 4cyl and 6 cyl version of the T9, possibly the V6 version had a larger capacity?? If you put more oil in as long as the input shaft and tailshaft input seals are in good nick shouldn't have leaks. There is no breather so pressurising the gearbox shouldn't be a problem.

If there is no breather then where does the pressure vent?

Of course there is a vent, look again at the top cover!
SFE

I've two type 9's, & both have vents in top cover.
I'd only fill as normal to the bottom of the fill hole. Excess oil may cause aeration & then poor lubrication.

I myself would happily overfill the box with the lower filler (1.4lt). Ford raised the filler hole to increase the oil capacity for a reason. But I'd use a good quality oil which would be a low foamer you would expect. In the past I used the Redline shock proof HD oil in my Quaiffe  syncro T9 and recently did a gearbox rebuild because of gasket leaks and found internals 'like new' my trusted mechanic said. However I always found the gear change rather notchy until the box warmed up. Note: no oil came out the vent after a good track (8000rpm) session either! Just changed to Castrol Syntrans which is considerably thinner and was impressed by the shift when cold. Hopefully it'll still lubricate and protect as well.
SFE

I think Ford started with 1.9litres and dropped the capacity later. I did some searching on Turbosport and found this (note, "Oel" is German for oil).
 
http://www.turbosport.co.uk/showthread.php?t=164685&highlight=5+speed+1.9+litres
 
 

Learn something every day? Might have to look for any visible date stamps/casting marks to confirm.
 
SFE

Certainly true, I hadn't noticed the box had a vent in the top cover, mind you I wasn't particularly looking for one when I installed it!

Interesting note talking about breathers.
 
Over Christmas I removed the engine and gearbox from my Escort panel van which has been my daily drive since April 1986.
It has always had the usual Mk2 Escort "healthy" gearbox extension oil leak. 
Despite changing the extension seal several times over the years the leak continued.
So this time I drilled out the small breather hole to 7.0mm and welded a 10mm OD ferrel centred on the hole and ran a rubber hose from the ferrel into clean air in the engine bay.
No more leaks (yet!). Not a drop.
 
So I'm tippin' the OEM breather hole is not large enough to cope with the internal pressure build up and forces the oil past the seals.