Next time you are filling it open the bleeder on top of the steering gearbox. If done with engine off till fluid comes out and full the system will need little bleeding.
If bleeding is necessary, I hook a section of MB hard vacuum line, with a rubber hose to couple, to the bleeder, open the bleeder and place the other end into the reservoir. Start the motor but **DO NOT** turn the steering wheel (you will blast the line off and throw fluid everywhere). The fluid will give up its air and clear fluid will flow through this into the reservoir.
As to your leak, the hole you describe allows fluid leaking from the spool valve to get out. Removing and sealing the spool valve requires the highest degree of skills involved in this box and almost total disassembly. It is not really hard but there are many ways to fail. I would find someone who can quote you a price without a book and let them do it. BTW we charge 4-5 hours labor and about $35 in parts for the whole job. I think a box of the car is about 2-3 hours labor to disassemble and totally reseal and set up.
The box doesn't have a flex coupling. The flange has two 8mm pins that go through two nylon bushings PART# 111 462 01 65. They are cheap replace every time the box comes off (or every few years which ever is longer). BTW the way it comes apart is to remove the safety clips on the 8mm pins (2x) and slide the coupling apart. Do not take the coupling from the shaft unless you have it well marked.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician