It needs a proper adjustment of control and modulator pressure. An easy job for an experienced tech.
If you would like to experiment, try removing the rubber cover off the modulator adjustment (rubber cap in center of modulator about the size of a nickel) and pull the key out of the castelated stops and turn it counter-clockwise one full turn.
It would also be appropriate to check that there is vacuum in the line to the modulator.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician