Just today I tested another of them John Deere
radial piston constant pressure source pumps I had already described here.
As I was re-reading the article I'd written quite a long time ago (I
published it in February on this site, but it had existed for more than
a year), besides many grammatical errors (sorry about that!), I noticed
that I'd completely forgotten to mention a very important detail
concerning these pumps, an error I am correcting in this add-on.
The very important difference of these pumps from
"normal" radial piston pumps, is the fact that they must work with a pressurized suction line,
which is the main reason there are two "suction side" fittings - the
suction side of this pump is, actually, inside the return line of
another transmission pump, which due to the existence of a radiator and
a return filter gets pressurized to a minimum of 2-3 bars during normal
operation. The suction check valves have springs, while common radial
piston pumps have free floating balls or use weaker springs.
It is very important to create "pressurized" suction
line conditions to test these pumps, because the suction line checks
weren't designed to work with atmospheric pressure oil. The pump will
cavitate, and you might come to a wrong conclusion about the pump's
efficiency if you don't find a way to "boost" the suction line.
One more thing - there definitely is an eight piston model around, which, by the way, has max. displacement of 24 cu. cm.