In the picture below, note the two highlighted parts. The cylindrical valve has 5 o-rings that go around it, and are internal to the gear box. These o-rings are notorious for getting hard and leaking. The leak will come from the weep hole on the bottom of the lower sector shaft cover plate. When this happens there is no way around removing the gear box and disassembling it.
If the outer sector shaft radial seals are leaking those can sometimes be replaced in situ, but can prove to be difficult. If the worm gear shaft radial seals are leaking (where the steering wheel shaft connects to the gear box) that will also require removal of the gear box.
The top of the gear box where the pre-load for the power piston to sector shaft is set has one o-ring around the metal disc and two copper crush washer between the nuts. This area can also be repaired in situ.
Bear in mind that replacing a few seals will usually cause the other old seals to start leaking, and that it may be worth taking the gear box out and re-sealing the whole thing.
Regarding seal kits.....Meyle and FEBI make kits, but I've had some problems with them not including all of the necessary seals in the kit. I've also experienced failed seals in 2-5 years time. I don't know what they're using to make their seals with, but it's apparently inferior to whatever Mercedes uses. That's where the catch is though. Far too many customers refuse to pay the price of the Mercedes kit, so I'm left with having to use aftermarket. The trade off is whether you want the re-sealed gear box to be leak free for another 15-20 years or 2-5 years.
Currently, the seal kit from Mercedes retails for $248 and wholesales for around $180. Aftermarket kits range anywhere from $30-$60. I understand wanting to save money, but given the aggravations in removing and re-installing the gear box I'd fork out the money for the Mercedes kit.
Beware of sellers offering rebuilt gear boxes online. Many don't take the whole box apart, and if they do they have no idea how to reset the pre-load. So you end up with a gear box that has too much play in it or is too sensitive. On top of all of that, they all like to charge a small fortune for them.
In essence, your gear box likely needs nothing more than a re-seal. Internal parts rarely, if ever, wear to any notable degree. However, you'll be better off long term removing the gear box and having the whole thing re-sealed. If you remove/install the box yourself, then make sure you:
1) know how to center the gear box and steering wheel for reassembly,
2) that you have the centering tool or some homemade equivalent
3) Renew the steering coupling/coupling bushings for between the gear box and steering wheel shaft.....never a better time than when the gear box is out.
