The W126 system uses a single pump that runs in two directions to provide both vac and pressure, and is electronically controlled. Not a good replacement unless you want to convert over completely, and that is probably more work than getting what you have to work properly.

Three are three things I'd look at first:

1. Check valve in engine compartment. If this is bad, vac leaks off. 45 min is way to fast, it should hold for weeks.

2. Cracked line in driver's door -- the main supply line runs here, and gets flexed every time you open the door.

3. Vac switch on driver's door -- these wear out and leak.

Go and get a MitiVac. REmove the vac line at the check valve and pump down. If it holds, the check valve is bad. You can also test this by pulling the line to the tank in the fender and pulling down -- bad chekc valve will hiss, and the vac leak will stop when you cover the other end with your finger.

If it still leaks, then remove the driver's door panel. Detach the lock and unlock lines from the door switch and check each one for leaks with the Mitivac -- doors should all lock and unlock smoothly, and each half of the system should hold vac indefinitely. If one side leaks, you will need to chase down the bad part (almost always a bad diaphram in a door or trunk element. The gas filler door is lock only, unlocks with a spring.

If both sides hold, apply vac to the switch after removing the yellow line and reattaching the others. If the switch leaks, replace it.

My locks work perfectly now that I replaced the bad section of line in the engine compartment. Not bad after 32 years.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!