Patrick,
This sounds good to me, it might mean that I don't need to take everything apart again. My front brakes have indeed been standing for a while (around 10 years), which means the rotors are prettty rusted, i.e. the metal is not shining ANY MORE, but this is only surface rust: Nothing more I've seen on cars standing still for 3 month on the roadside. I have rebuilt the callipers a month ago, and the car is now stored in a dry place, so they should be working fine. Do you think rust on the rotors could be enough to prevent the front brakes to grab? Or does it need frozen callipers to do so?

Next action on the list will be disconnecting the rear brakes to watch the braking power on the front ones. How many miles did you need to solve the problem? So far I've driven around 20 miles without improving anything, but I admit I did not brake too hard (didn't want to take a field trip...). Since the car is not yet insured/registered, I can only go round the block at night when no cops are around. Will this be enough?

Thanks for your help!
Séb