Brake Booster, master cylinder or Vacuum Pump? 300sd

[cousindave76]

I'm sorry but this will be a long post. However, as you know, the symptoms of these lovely, eloquent MB automobiles can not be simply describe in just a few words.

I have an '83 300sd with booster problems. I'm trying to figure out if it's the break booster going bad or the vacuum pump. Or possibly the master cylinder!

Here's what it does... When I press the brake pedal down, it initially feels like it has plenty of stopping power but, when I try to press any harder it's like pressing it against a brick wall. She'll do no more than the initial amount I pressed... i.e. if I'm approaching a stop sign and start applying the brake I have two choices... 1. wait until I'm almost there and hit the brakes hard and it stops fast and hard or 2. press the brake with plenty of time to spare and roll up to the stop sign nice and slow while pressing very hard on the pedal to get it to come to a complete stop.
I've also noticed while sitting at a stop light that is on a slight down hill(causing me to apply extra brake power) that my climate system fades away. The center vent vac modulator does not have enough vac left to stay open.

So that's what it's doing... Here is what I've done to diagnose the cause.

I really don't know a way to check the brake booster itself so I'm hoping that eliminating or identifying the problem from other components in the system with narrow this down.

I've bled the brakes exhaustively... I promise there is no air in the break lines.
I felt no improvement in my brakeing after doing this.

I checked vacuum. I don't have a way to check the vacuum directly off the pump due to fittings I don't have to adapt to my vac gauge. From the main/brake booster vac line I disconnected the vac line that goes to the shift modulator and components. I hooked my vac gauge up at this point and started the car. The vac slowly but eventually climbed up to 23" hg. A good reading I thought but I'm not sure how fast it should get to that reading. Then I pressed the brake pedal down and noticed it sucked all my vacuum down to about 5" hg and sometimes less(I assume that's why I lose climate control while sitting at a stop light). When I release the brake pedal the vac slowly climbs back up to 22-23" hg. ?????

is this my booster, MC or vacuum pump?

Any ideas? Any other testing I should or could do? Your input please!

This is my only car... I would love to be able to stop when needed... for my safety and mostly the safety of others... please help me figure this out.