A few weeks ago I picked up an Agfa Isolette II, a wonderful German 120 folder from the early-mid 1950s. It all seemed in relatively good condition until I did a bit of research & found there are a few common problems which plague these cameras: the bellows leak, and the focus ring gets stuck.
Fortunately, the first was solved by a liberal application of liquid electrical tape, and being super gentle when folding/unfolding the bellows (if I fold them up at all, which I try to avoid).
The second wasn't so urgent, as the focus ring still moved with a fair bit of pressure. Bit annoying when you want to take up-close photos as well as landscape shots, though. So last night I thought I'd have a go at fixing it myself. Virtually all I did was use a very fine paint brush (and a bit of folded card) to scrape away some of the Agfa "green gunge" (the grease used to lubricate Agfa cameras tends to turn into a thick, sticky green gunge over time), then used the same brush to apply olive oil to the thread. Mind you, I used the tiniest amount of oil possible, as I have heard that the oil can get into the lens or (much worse) onto the aperture blades, causing major problems. Then I just worked the focus ring a few times between close-up & infinity to get the oil into the right spots.
It seems to work fine now, quite smooth - and no oil on the lens or aperture/shutter.
NB: I take no reponsibility if anyone tries this method to fix the focus & it turns out badly! I was doubtful it would work at all, I'm pleased with the result but still expecting I've fucked up somehow that I just haven't realised yet.