The hydraulic pump itself looks to be bolted to the outside of an upper flange face on the transmission. The pump itself is part 010 and should have a rubber suction hose running to it (not shown here). I've heard people on TBN talk about a leaky suction hose to the hydraulic pump. Weathered and cracked suction hoses are a common problem with hydraulic pumps.
To help you get oriented, the first is a picture of the HST transmission case. The snout sticking out on the lower right of the illustration is where the left rear axle plugs into the transmission. In the second picture, the rubber hose is visible and also the HST filter. You can also see the cleanable screen part 030 in the second picture. That screen has to be taken out and cleaned every few hundred hours or it will plug and nothing works.
rScotty