Gearbox oil leak - possible vent blockage?

[Chrome or rubber]

I have just completed a 1,500 mile tour of Scotland and needed to call into a garage on two separate occasions to use their high-lift to top up the gearbox oil and now I'm home the same is required again, so basically 3 oil top-ups in only 1,500 miles. Each top-up required 2 litres of oil.
The car is a 1979 MGB with an LH overdrive (gearbox & O/D total oil capacity of only 3.4 litres).
The bulk of oil appears to be lost via the rear seal, but even if the seal is badly damaged the loss of 1 litre per 250 miles seems excessive, could the vent be blocked and is there an easy fix without the need to pull the gearbox out of the car?

[Vic Butler]

I've had a look at the parts list and no vent is shown. Possibilities are the speedometer drive, overdrive isolator switch, reverse light switch or the front oil seal. With that amount of leakage can you see any likely place it's coming from?

[Chrome or rubber]

Thanks Vic,
However, the gearbox does have a breather and the part is listed as item 78. The part number being the same as the rear diferential breather.
However, it is mounted into the gear change extension and I suspect a right nightmare to reach with the box still installed... that being my question I guess, can the breather be cleaned/changed with the gearbox installed in the car?

[Vic Butler]

Took that to be a stud.
It might be possible from underneath but there's very little room. Can you get your hand to the breather and see if it's oily?

[Charles Farran]

You should able to reach the breather ( if this is causing the issue) which is forward from the overdrive switch (22) on the top of the gear box.
When i changed the gear box harness i could only reach this switch by dropping the gear box using a hydraulic jack under the gear box . With the car on axle stands, you remove the 4 bolts holding the gear box cross member with the jack ready to support & then lower it a couple of inches.( My car is a 1980 so had already removed the gear lever surround etc. - there is enough flexibility in the front engine mounts & the prop shaft UJS not to create problems elsewhere) , the gear lever & the additional metal plate surround that created enough space for me to get my hand in to put the leads on the switch. (I didn't disturb the breather but i suspect you would have room &access to reach this from under the car with the gear box lowered - but i would check you can't already do this before lowering the gear box).
Cheers,
Charles

[Chrome or rubber]

Thanks Charles,
Mine is a 79 so, should be same or very similar to the 1980 version.
Good to know the box can be lowered sufficient to get at the breather.... many thanks.