First off, you should be aware that by '91 there were two, completely independent vacuum systems. There is an electrically powered vacuum pump usually located under the right rear passenger seat. This supports the locks and interior components, and is the likely vacuum source for your headrests.

The engine-driven vacuum pump supports the engine compartment, HVAC, and transmission, in addition to the brakes. The 'brake side" supports only the brakes and the shut-off servo, everything else comes off the accessory port. The two ports are independent.

Before you start diagnosing, you need to invest in the proper tool...you'll find that most MB tinkerers use a Mityvac, which is a vacuum gauge and pump in one unit. You can use a simple vacuum gauge, but eventually you will find yourself compelled to use the right tool.

The way you diagnose is to start at the source point for the component you're trying to check. Say you're trying to check the transmission modulator. The source is the accessory port on the engine pump. You will find that just after the port, there is a multi-way connector. Disconnect one leg, and plug in your Mityvac. Use the Mityvac to pull a vacuum. If it holds, the leak lies along the disconnected leg, while the remaining legs on the vacuum run are fine. Using this technique at every branch, you will find the leak with no more than half a dozen vacuum pulls. It's that simple.