Brake light is on sometime and goes off when restarted. Computer has insufficient vacuum.
After the car was sitting overnight and car off, the brakes seems to have vacuum as indicated by 3 brake pedal pumps before being hard to push down.
With engine running, mighty vac has 29 in Hg vac and goes down with each rapid brake pedal pump (1-2 on depression and 3-5 on release of pedal). 3 pumps when down to about 15.
Takes about 30 seconds to go back to 29 with pedal not pressed.

I tried to find a vacuum leak by have car on and using brake cleaner to find the leak to no avail even on the inside of the boost plunger.
I removed the check valve on the the vac pump and tested by mouth -- works.

Anyone else have this problem?
Mech thought the pump (2762300265 or 2762300065) was bad. It looks very hard to replace being almost no space between pump and firewall. Any tricks to remove the pump?
What should be used to regrease the booster plunder? Dow high vacuum silicone grease or plumber's silicone grease?

How to reattach the booster's plunder boot near the brake pedal?

Thanks,
Dave
W212 2012 e350 4-matic

What DTC are you getting ?....to indentfy that the brake light is warning due to lack of vacuum at the brake booster ( sensor is there at brake booster ).
Brake light can also indicate low brake fluid level.

This is a normal vacuum pressure useage of the brake booster
Remember, my sensor uses absolute sensor.... so zero PSI is 29.9inch of mercury or full vacuum.
If 14.7 PSI meaning zero Inch of mercury = atmospheric pressure, me at sea level.

> FIRST THING...
Dave... you went down a proverbial rabbit hole fixing your vacuum booster/pump without first diagnosing the issue.

> SUPER SIMPLE:
you or your local MB specialist can use an OBD scanner to read exactly the brake booster vacuum level under 5mn
Besides scanners can also get the list of fault code descriptions to help troubleshooting work.

Once you have a target, you can go after fixing the condition.

> VAC PMP OK:
Pump may supply limited vacuum output caused by high crankcase pressure. Don't replace the pmp!! You're not alone, others report popping dipstick.

What is your current mileage ??

FYI... today after 26 days unused, I had enough vacuum left in the booster to squeeze brake pedal to start. Booster used to go flat in as little as 2 or 3 days with higher crankcase pressure.

Thanks for the replies. It seems like I have good vacuum that is close to the 0.1 psi (30 in Hg relative on my analog dial). The video has the vacuum dropping to half vacuum after 1 or 2 pumps, My vacuum drops with each pump until half at 3 pumps, then dropping to 1/3 vacuum with more pumps. The video has a 20 plus seconds recovery and I have about 30 seconds recovery. I don't think the gauge is accurate/precise enough to between 29 and 30 in Hg vac.
The brake light didn't go on during my testing that prior indicated insufficient vacuum on SNAP-ON scanner/computer. The mech just told what the codes mean and not the numbers. Maybe I just had a gremiling in the vacuum sensor (going bad). The light didn't go on during my testing. Maybe a leak to track down or pump getting old (2012 with 75K miles).
I will apply a little wipe of plumber's grease to the plastic plunger. The Dow High Vac Grease will be too thick in cold weather.

Happy Thanksgiving.
How how do I get the boot next to the brake pedal back on?

You must google and see utube for similar type your MB is using
I have not touch my brake components that far yet for the leg room area.

your testing shows that your vacuum supply may be somewhat marginal.

Your low 75k mileage make the pump failure to be very unlikely.

The vacuum pump output is shared between two check valves. The large one for brake booster is kept reliable and the small one is often found defective or blown off.

Beyond that this is not a high volume unit but rather a deep vacuum. Let me try to capture my vacuum at idle for your reference.

After 3 days the vacuum held as tested by 3 good brake pumps that had vacuum assistance.

Well, the cleaning of the one way valve at the pump(1) has cause the recovery after rapid brake pump be restored to 28 in Hg in about 10 seconds (was taking about 30 seconds before). Where is the second check valve? There's the booster, a Y connector, larger pipe to another Y connector with one larger pipe going to the pump's vac valve on the pump, and the other smaller pipe to the front of the engine.
The light has not returned so far.

Now it's getting the boot back on. -- maybe removing the brake pedal assembly to get some room to get it on. The only video I found was on a Volvo that said it just slips on using two hands. I can hardly get one hand up there with the pedal there.

(1) I sprayed brake cleaner into the pump to dissolve any sluge (small oil return that is known to clog) and then re-oil the pump with 1 to 2 mL of oil down the same hole.

Happy Thanksgiving,
Dave

at 4:30 minutes in video on the amount of vacuum (-0.8 to -0.9 bars = 24.4 in Hg, I have 28 - 29) and recovery times (10 sec). He was ordering another sensor, so the reading may be off, just the recovery time is important for my test.
My Mighty Vac has a leak, so I can't measure vac over time.

Still messing with getting the boot back in place. Any one else with a W212 or W218 that has a close up picture of the boot connected?
Here's a pricture of mine. Is the boot with the tabs folded outwards or just attached by the tabs?

Also on prior question from CaliBenzDriver, the PCV is working well. There's negative pressure at the oil cap. I have a mechanical pump, no aux electric pump assist.

See if this can help......

This is the best image I have for my brake booster



See these too.........


and



Always check ebay for used parts, often high quality images available.
Example : https://www.ebay.com/itm/355194776937

and
https://www.ebay.com/itm/225750627151


Good luck

It seems these are how they are arranged and their function , 3 items below :



Guesstimate :

A is more of a dust seal. Like our shock absorber dust boot.
B is the VACUUM seal which was replaced in the URO video. Dynamic seal, takes the sliding in-out friction workload.
C is part of the VACUUM seal assembly of the booster fat body. Static seal.

Did you get a new boot? The filter material on my boot came off both on the outside (as you picture has) and on the inside vent slots (not shown). Where to you get the filter material?
There's also very little space between the plunger seal and neck of the booster for the dust boot to hold on (my exposed C is very narrow).
Thanks,
Dave

I did not do anything to my brake booster , it is fine.....I just took photo in 2021 for wiring thru firewall and I documented what are the stuff there hidden.