Hi Phil. My dad in the USA has been rebuilding them for 1800s, 120s and 140s. Can you tell where the leak is coming from? There's a seal on the bottom Pitman arm shaft, and one on the input shaft. Then the top cover of the box and the front cover have gaskets. If the box doesn't have significant play or lumpiness and hasn't been run dry or with water ingress, you could probably get by with just replacing the seals/gaskets. I can try to upload a PDF (see below) of an article my dad wrote about rebuilding the boxes that you might find useful. Note if you would like to contact my dad his email is garysievert@gmail.com (it has a typo in the attached pdf) George Minassian can probably source most of the parts, or you can source yourself if you get the parts exploded views.
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