Do you have a vacuum gauge? If so tee it into the black line that goes over the back of the engine to the vacuum modulator.
The vacuum should be high at idle and drop to zero at full throttle. We verify this by driving it. This requires a long line on your vacuum gauge.
The clear/white vacuum line we use can be bought for less than $1.20 a meter from a MB parts department under part# 000 158 14 35. A three way rubber tee is available under the number 117 078 01 45. It lists for $3.60. With 4 meters (maybe only 3 but whats another buck)of line and a tee and vacuum gauge should do these tests.
Bosch Master, ASE Master, L1
33 years MB technician