Modulator pressure is adjustable. The 420 and 560s have the additional common fault of the vacuum line to the modulator falling off the vacuum port. The port is hard to see but the line can be seen with the air filter off looking down the backside of the intake manifold. The large vacuum line going to the brake booster starts in this area. There is a banjo fitting and the small port is in that area. Look for the white plastic line passing under the crossover exhaust pipe. That is the line to the modulator. Be sure it is plugged in.
BTW the complete symptom package is a rough idle (vacuum leak) with harsh shifting.
Bosch Master, ASE Master, L1
33 years MB technician