Ive resealed the pitman arm seal with the box on the car on several 126s and the box on the 107s are the same unit. If there isnt anything in your way remove the pitman arm and unbolt the square plate thats at the bottom of the box where the pitman arm comes out. Pry it out and replace the large oring on the inside (it seals to the ptman shaft and does most of the sealing work and replace the outter seal that is held in with the circlip. Installing that outer seal is easy as all you have to do is push it in place by hand and put the clip back in. Now lube it up all nice with vasaline and reinstall it back onto the box. Take note of the position it was in when it came out so it goes back in the same way. Its been a while but Im almost sure it has a small oring between the plate and housing body like the top plate but maybe Im wrong cause its been at least a year since the last one I did. In fact it should be an inner oring and an outter oring and the seal where the clip goes. When you do it you will see its not rocket science and isnt all that hard. The hardest part is getting off the pitman arm.
Well what more can I add to help? Are you guys familiar with adjusting the play in the steering gear? OK at the top of the box is the adjustment screw with a 19mm lock nut. If you see a lot of theads showing (like 7 or 8) its been already adjusted a couple times and the gears are worn inside the box. If you only see 3-5 threads showing then the gears have room for more adjustment and the box isnt all worn out.
Start by using a deep 19 mm socket to break loose the nut. Just crack it loose and then hold the nut with a shorty box end wrench. Now using a 6mm allen on a long 3/8 drive extension turn the screw counterclockwise untill it gets tight. Now turn it back in at least 1/2 turn and holding the allen in position tighten the locking nut. Test drive the car and after making a turn if the steering dosnt return to the straight position on its own then its too tight. Go back and screw the adjustment screw in an additional 1/2 turn and test drive again untill its free enough for your steering to return as normal. Otherwise you will make a turn and when you let go of the wheel the car wants to continue turning! For you to better understand this adjustment the adjustment screw is attached to the top of the pitman shaft/gear and as you turn the adjustment screw counterclockwise you are pulling that shaft upwards and making it mesh tighter with the steeering shaft gear because the gear has a tappered cut. I hope I havnt lost you so far and you understand what Im trying to explain. Now another thing exists as these steering gear boxes get old the gears wear and you cant always get rid of all the play by adjusting it. For example your best adjustment without making it too tight still has 1-2 inches free paly at the steering wheel and thats as good as it gets for that particular gear box. So what do the sneeky gear box rebuilders do in order to give you the ILLUSION that there is no play? Ill tell you what they do. When they rebuild the box they add a little extra pre-load to the bearing at the top of the box where the steering shaft is. This gives it a slight tight feeling faking you into thinking the box isnt worn out but the 1-2" inches of free play at the steering wheel is still there. Ive done it before for those customers who say they are soon selling the car and dont want to spend the extra money. You know the cheap asses that still own the car a year later....hahaha
Now that preload adjustment can be tricky and is best left alone or only for seasoned mechanics that are real familiar with it. So all in all if you see the adjustment screw showing lots of threads dont waste your time. Just get another gear box to replace it with. And look for one that isnt showing much threads above the lock nut. I hope this was more help than confusion. Good luck guys and sometimes you will surprise yourself with what you can do on your own once you get into it. But if you get in trouble or over your head its always good to know a good technician that can bail you out. Good luck and good motoring my Mercedes brothers!