If you donut have a tool to change the rear brakes is there maybe a other what to release the cylinder on a Volvo s40?

you can use a c-clamp and a pair of needle nose. turn the c-clamp in and put the ends of needle nose in the grooves on the caliper and turn at the same time. Its a real pain but it will work. If you have a harbor freight tools by you, you can purchase the tool for about $20.00 dollars. Even if you use it once its worth it.