Pete,
Many thanks for your quick answer. I remember that you already mentioned the crucial point of not swapping leading & trailing shoe in a reply to one of my earlier posts. I also recall paying special attention to that when putting my brakes together after I read your post. There is little chance that I made that mistake, especially with George's picture in front of me.
Does such a swap cause a premature locking of the drum, and if yes, why? (this is just natural curiosity...). So maybe I got that wrong, but as I say, there's maybe 3% chance that this is the cause of my problems.

On to my brake valve. The one I've used is shown in the Volvo parts catalogue. It has a bolt on one end that looks & acts as an adjuster. When I say I hear it opening, I mean that with the motor off, while pushing the pedal, I can hear a spring-like noise when the pedal is 3/4 way down, and I can feel the click whith the foot. It's very light, though. In my opinion, this comes from the opening valve. Maybe I'm wrong...

What about the cylinders now? Can you confirm that the brake-valve equipped cars have smaller cylinders than the 1' ones used on the "unvalved" cars? Or should I get 1' cylindrers instead? By the way, when installing the cylinders, I noticed that each new small one had a spring pushing the pistons aprart, whereas the old ones didn't. Does this make a diference?

Well, sorry to bother you with that, I'm trying to identify the problem & think of everything I can before taking the drums out again... Thanks for your help!
Regards
Séb