Bedford CF Auto Box

[likaleica]

I'd be grateful for some informed advice please on the Vauxhall three speed automatic box in my 1983 Bedford CF camper. While towing a trailer with my classic car aboard, the box developed a fault - holding on before changing up the gears. It now changes 1st - 2nd @ 30 mph and 2rd - Top @ almost 50 mph. The box downshifts properly.
I'd just got the van back from a service by a rather hamfisted garage and I think they MAY have neglected to top up the transmission fluid - but as it's rather difficult on these to establish the correct dipstick reading I'm not sure. I must confess that in 3 years (though only about 10,000 miles of motoring) I've never cleaned the filters or completely changed the fluid. As I'm not familiar with auto boxes I decided to "leave well alone."
Have I fried the box and it needs to come out for repair - or is there another cheaper solution ?
Many thanks guys - Simon

[likaleica]

Many thanks for all the quick replies fellas ! It's the original slant 4 2.3 petrol engine and GM 3 speed auto box. The link to the manual would be warmly appreciated. Cheers

[likaleica]

Thanks again guys. I tried to sort the kickdown cable but the Haynes manual instructions are so unclear I didn't succeed. Does anyone have a simple description of what I need to do please?

[likaleica]

At long last the auto box is sorted and the likely problems were flagged up by many of you experts out there - so thanks very much for your help.
The vacuum pipe from the engine was split; there was a heavy transmission fluid leak (masked by a massive oil leak from the rear crankshaft oil seal)and these combined to make the box misbehave. I am hoping that now they're rectified I've done no lasting damage to the auto box - time will tell.
Thanks fellas.

[likaleica]

Unfortunately the original rope oil seal has just been renewed - but I'd be very interested in knowing what rubber seal you are referring to for next time !