You can use a propper vacume tester, this evacuates the line/s and will show you via the gague if the line is / is not holding vacume.

They are obtainable @ most auto access shops.

Or wet your finger, disconnect both ends, sealing one with de finger and suck on the other end. Block with de tounge and you will feel it either hold onto the tounge...or leak. Cant guarentee the taste though.

Carefull replacing the air/mass flow meter unessesarly. they not cheap.

I've got a W124 300e with a 103 motor, same or simmilar cruie control issues.

Just had the volage regulator changed(it houses the alernator's brushes), and now due to wear, over 230K Kms, have just had the alternator o/hauled.

This is supposed to fix 99% of the problem, re cruise control n/w or intermittent... but alas seems I'm not that lucky.. Battery is new and of correct size.

So the autoelec has just inspected and resoldered all the cruise CU (control unit) incase of poor contacts etc, but he found nothing burnrt out, the pc board was fine and guess what?? It still doesn't work/may work when it feels like it???

The CU is located behind the drivers side kick-panel, incase you need to acess it & do the same.

Suggest you try and follow what I've done so far, it wont cost much and you probaly find your alternator/electrical system needs a breathe of fresh air/repairs.

Best to check one issue @ a time and start @ the begining and work allong those lines I've mentioned. DONT just change the Air Flow, there is also a Zenit type 12v Bosch relay, which controls ALL the voltages. If this plays up, like mine did, it will appear as an inoperative A/F meter or sensor.... so beware he who changes part without correctly testing them 1'st !!!!!!!!!

C how you go & keep us up to date
chow,
Graham