Hi Henk,
GAT, there are a few things that will make the brakes stick. The first is the calliper pistons or sliding pin seizing. This is not your problem as releasing fluid helps.
The next is the return hole can be blocked. Often this is just there is no freeplay at the lever & it is slightly pushing the mc piston in enough to close off the return hole. Dodgy aftermarket levers will do this for free if you’re lucky, though the barkbusters are worth a look. Also take the cap off & check the rubber is sitting properly & isn’t ripped. Clean the vent channel in the cap. It will be blocked with white residue.