Hello coltom welcome to YT! First off is it leaking
engine oil when the engine is being worked or
hydraulic oil when hydraulic pressure is being used? I
will assume the latter. The Ms obviously did not
originally have live hydraulics and the Supers so not
show a break down of the pumps. I have attached the
manufacturers online parts catalog for a Farmall 400
they use the same pump. In the Hydraulics section,
look at subsection 371 and 373. Those sections will
help you ID which pump you have. The one version has
a block that bolts up under it and it is possible the leak
you have is from them. They can easily be replaced
with common hydraulic orings. YT has seal kits for
them, find your pump number and call them.
Just FYI, that link will also allow you to access the
parts catalog for your M, just type Farmall M in the
search bar. Also FYI Many parts are unavailable and
many that are have outrageous prices. Also their site
does not work well when viewed on a phone, or at least
not an iPhone.
CNHI Farmall 400 online parts catalog