I have the following situation:

- if I drive on the highway and hit the brakes all is working fine
- if I'm at a red traffic light waiting then the brakes become weak and the car pushes forward unless I press the break pedal really hard (hard pedal but not going to the ground)
- break fluid is normal / no leaks, front discs / pads got changed recently, power bled the system

Today I did the following:
I hooked up a vacuum gauge to measure the vacuum of the line that goes to the booster.
Here are the results:
- vacuum when the car starts goes all the way from zero to 19 Hg (this takes some time - maybe 20 sec). This is in P.
- when I hit the brakes vacuum goes to 15 Hg,
- If I maneuver the car and hit the brakes several times then vacuum goes down to 5 Hg, if I hit the brake pedal then very hard - vacuum builds up to about 17 Hg but brake pedals stays hard

Now my question is:
- is the brake cylinder bad?
- is the booster bad?
- something else?

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'80 300 SD, 300k

I think that is classic booster bad symptoms...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Could be leaking vacuum system or even a worn pump.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Boosters go bad sometimes. Worn pumps happen more often, especially the diaphragm kind (not sure if you have one). Vacuum leaks are far more common.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

To see if it is the Power Brake Booster when you first start up. Remove the Vacuum Line Connector at the Brake Booster and have someone block it off with their thumb or other finger.
Start the Car and see if your Vacuum Reading improves. If the Vacuum Reading is till as you previously described it is not the Brake Booster.

Remove The Vacuum Line at the Vacuum Pump and unscrew the Check Valve that goes into the Vacuum Pump and check it to see if the Parts are still all there. If the parts are gone or have issues replace the Check Valve.

If the Check Valve Parts seem OK clean out the Check Valve with WD-40. Do not use Brake Cleaner as it can soften and make sticky some Plastic Parts. Re-install the Check Valve and see if it restored your Vacuum.

It also would be great if you could check the Vacuum at the Vacuum Pump byitself.

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84 300D, 82 Volvo 244Gl Diesel

Here is the solution for the bad braking performance
-> brake booster & master brake cylinder
- The dash has to be removed and the air vent line to the left driver air outlet.
- The upper left screw of the booster is a bit a pain but with long extensions it opens. Also the screw that goes through brake pedal needs to be removed.
- disconnected the master brake cylinder from the booster but left all brake fluid lines connected. Then bent the brake cylinder carefully away to make enough room to pull the booster
- The old brake booster had brake fluid in it. That indicates that the master brake cylinder is bad.
- I put the "new" brake booster in (I had one from a w126 which is exactly the same as in the w116)
- Since I didn't have a new brake cylinder handy I connected the old one
- Braking performance improved slightly.
- unfortunately I must have pulled / damaged one of the vacuum lines since my doors are no longer locking and the trunk is closed at all times... well another project
- I ordered a new master brake cylinder from Metelli
- I put it in last weekend: took off and cleaned the reservoir,
- removed the brake lines,
- removed the screws on the booster and pulled the old master brake cylinder
- instead of "bench" bleeding I connected the new master cylinder to the booster and also "pressed" in the reservoir
- then I connected some rubber hoses from the 3 outlets of the master brake cylinder to a plastic cup
- then I filled the reservoir with brake fluid
- I pumped a couple of times and you could really see the bubbles of air coming up in the reservoir
- also the cup filled a little bit with brake fluid as expected
- then I connected the brake line for the rear, then I pumped the brake pedal to make sure no air is in, and then connected the other 2 lines (pumping in between)
- after everything was connected I went for a test ride and the brakes work perfectly
- I have a power bleeder but since the brakes work so well now I think, it's not necessary to bleed at this point

thanks for all the feedback!

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'80 300 SD, 300k