Hydraulic motors have leakage and will not hold position. You need to add a brake. The case drain is there to accommodate this leakage. Some types of motors even have holes drilled in the pistons for lubrication purposes.

It sounds like the correct type of motor for your application is the Char-Lynn type gerotor motor. https://www.eaton.com/Eaton/ProductsbyMarket/Agric...

I don't think you really want to do it that way. You may think you do, but you don't. I think you really want to do it with a stepper or servo motor with a colossal low-backlash gear reducer on it. No hydraulics. No leakage. Holds position when you want it to. Accuracy as accurate as you want.

I agree with Brian, a proper gearbox and motor is the way to go. Stepper, servo, or if close enough is ok, an induction motor with VFD or even a DC motor may work. Hydraulics tend to not work the same way hot as they do cold, internal leakage in the pump and motor will change with temperature. Unless the hydraulic motor is designed for very low rpm, it may not even start moving with such little flow. Oh, and look at the prices for hydraulic servo valves and amplifiers if you still think you want hydraulic.

Sounds like you need a slewing ring or worm gear to drive this. Hydraulic motor inout and the gear ratio slows it down, the worm gear kind of acts like a brake

We recently sold $250K of hydraulic motion control to India for radar dishes. I have no idea how they are going to be implemented mechanically except they are using closed loop control. They could use hydraulic motors for full rotation or a simple linear hydraulic cylinder connected to an arm of the rotation angle doesn't need to be that wide. The controller can convert linear to rotational positions or back. There is no need to worry about leakage because of the closed loop control with servo quality valves. There is no need for a brake but I am sure they have some sort of POCVs/blocking valves so the dishes don't move without hydraulic pressure.
I concern we have seen in the past is a notch filter to compensate for dish resonances when the wind blows. These can be implemented in the user program.

If the dish isn't too big you can use a Moog Flo-Tork
https://www.flotork.com/content/dam/moog/literatur...
The flotork converts linear motion to rotational. We/our distributor have used flotork and our controllers on test nose wheel landing gear test systems.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/
IFPE Hall of Fame Member