Excellent advice. Look here for procedures:

http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Index/Groups/43BrakeBooster.html

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84 300D, 82 Volvo 244Gl Diesel

The new '82 SD I brought home today does the same thing. I just don't think the vac pump has the guts to maintain vacuum at idle when you pump the brake pedal.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

On my Volvo diesel.
If you are sure your Vacuum Pump and Power Brake Booster are working correctly and you want to be able to pump your brakes more and still maintain enough vacuum to work the Booster; you can try adding a Vacuum Reservoir in your system. It is only a hollow container that gives you more vacuum volume to work with.
I have been using a large about 1 gallon “tin can” that I drained the contents out of with out using a can opener to take the lid off. I poked holes in it and threaded brass hose fittings into the hole (threads coated with JB weld). It allow me to pump the brakes more and still maintain normal braking.
It has shocked several people who have happen to see it when I had the hood open (it is an unpainted restaurant size tomato sauce can).

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84 300D, 82 Volvo 244Gl Diesel