Servo-freio = "service brake". These are the regular four-wheel hydraulic brakes not the "parking brake".

?diminishes the effort of the driver during the braking?

If you are saying that the brake pedal moves increasingly toward the floor when pressed, that is a problem. The pedal should stop partway down, and not go any further down. If is moves continously towards the floor, there is a leak, probably internal to the master cylinder and it should be replaced for safety.

If the brake pedal goes too far down (but stays at that position), the rear brakes need to be adjusted. It is fairly easy to do:

Adjust rear brakes:
1. Put car in nuetral, block front wheels
2. Jack up one of the rear wheels. Block it to prevent falling
3. Spin wheel by hand. Turn the 1/4 inch square adjuster (originally covered with a black rubber cover) clockwise until the wheel cannot no longer spin. Use a little force to see if the adjuser will go tighter.
4. Back off the adjuster until the wheel spins free. The adjuster is square, so it needs fine-adjustment. Better to be tight than loose, as long as the wheel spins free.
5. Repeat for other side of rear