There are three lines,
1. Main vacuum feed line
2. Line to egr valve
3. Line to the vacuum bleed valve on the top of the IP and the tranny.

The main line will always be pulling vacuum but the other lines will not until the throttle linkage moves and opens this valve. When it opens you will then read vacuum on the other two lines.

Since this system is tied into the bleed valve on the back of the injection pump when you go full (wide open) throttle on the car the bleed valve allows air into the system and thereby reduces the vacuum which in turn closes the egr (robs power when open) and makes the tranny shift harder to avoid slippage at full throttle.

It only has one purpose and is not usually prone to breakage internally but the little foot that rests on the throttle linkage usually breaks off.

On the 1980 cars there is no bowden cable for transmission adjustments just vacuum and it is a real balancing act with an older car to get the vacuum, modulator and throttle/power settings in sync to give you the perfect shifting car...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique