No brake boost at idle - PeachParts Mercedes-Benz Forum

bgkast · #1 08-01-2008, 01:14 AM

Tonight while descending a member's rather steep driveway in neutral I discovered that I do not get vacuum assist for my brakes when the car is at idle. Once the car is revved up the vacuum assist works fine, but when it returns to idle you get maybe 5 pedal cycles before the brake pedal firms up. Thinking my VCV may be at fault I tried repeatedly pressing the brakes while engine braking down from highway speeds, but they seemed fine. The vacuum locks and shut down work fine, and I checked my vacuum at idle a few months a go and I believe I had 20" of vacuum. I will check it again and report back. What else should I check?

Cervan · #2 08-01-2008, 01:16 AM

Quote:

Originally Posted by bgkast

Tonight while descending a member's rather steep driveway in neutral I discovered that I do not get vacuum assist for my brakes when the car is at idle. Once the car is revved up the vacuum assist works fine, but when it returns to idle you get maybe 5 pedal cycles before the brake pedal firms up. Thinking my VCV may be at fault I tried repeatedly pressing the brakes while engine braking down from highway speeds, but they seemed fine. The vacuum locks and shut down work fine, and I checked my vacuum at idle a few months a go and I believe I had 20" of vacuum. I will check it again and report back. What else should I check?

Try isolating everything off, and only connecting brake booster. If you still have this problem maybie you have a vaccume pump thats failing.

blackestate · #3 08-01-2008, 08:52 AM

Wow, What a ride that must have been! mut the vac gauge on the line, and test vac, and test while pressing breaks repeatedly. With the rest of the vac system disconnected. I would guess a cracked hose?

kerry · #4 08-01-2008, 10:00 AM

The booster consumes a lot of vacuum. I wonder what is normal at idle? I'm guessing most of us have not used our brakes under these conditions.

bgkast · #5 08-01-2008, 03:59 PM

I'll try it on the 300D tonight.

winmutt · #6 08-01-2008, 04:46 PM

Check valves and/or vac pump rebuild time. I have a rebuild kit for the vac pump I'll never need. I'll sell it for under cost.

blackestate · #7 08-01-2008, 07:29 PM

I have had no issues like that on either of my cars. Suspect there is a leak/pump issue.

Cervan · #8 08-01-2008, 07:37 PM

No power steering and no vaccume (means no brake assist) is alot of fun :O

Jeremy5848 · #9 08-01-2008, 08:10 PM

Sounds like a leak. The vacuum pump moves more air as it is pumped faster as the engine runs faster. As cervan says, remove all vacuum consumers except the brake booster, see what happens. If it's OK, you have a leak. If it's still bad, you have a leak in the main line or a failing pump.

bgkast · #10 08-01-2008, 09:27 PM

OK here are the test results:

In the 240D 3.0 turbo (the car that I had the problem with yesterday) I disconnected all vacuum accessories from the main vacuum line and hooked my vacuum gauge to the main line. I started the car and vacuum built up to 20" in about 10 seconds. Pressing on the brake pedal would drop the vacuum and pressing on it 7 or 8 times in a row dropped it to 0, where it would start building back up again.

In the 300D I did the same test. The vacuum took much longer to build to 20" (maybe 90 seconds). Pressing on the brake pedal would drop the vacuum, but not nearly as quickly as in the 240D. I estimate it would take 30 cycles of the pedal to drop it to 0" The 300D also seemed to have a firmer brake pedal feel than the 240D.

Both cars seem to have a “tight” vacuum system and will still lock and unlock after several days of not running. What should I make of this?

kerry · #11 08-01-2008, 09:35 PM

I think there are different vacuum boosters on early models compared to later models. Perhaps the change was made to reduce vacuum consumption and avoid the problem you experienced.
My attitude would be to ignore it until it happened again.

winmutt · #12 08-01-2008, 10:39 PM

Check valve. Not as in check the valves but the part. [img]http://www.***************/secure/PartImages/0049971872.jpg[/img]

bgkast · #13 08-02-2008, 01:47 AM

If the check valve was bad I don't think it could pull 20" of Hg, no?

winmutt · #14 08-02-2008, 09:46 AM

I do not believe that to be the case. I had the *exact* same problem on my old engine and never fixed it. Everyone said it was the check valve.

DeliveryValve · #15 08-02-2008, 11:28 AM

Check valve might be it, but you might want to further investigate the brake booster. Try to see if the vacuum holds at the booster during different levels of pedal travel. Each point the booster should be holding vacuum. If not then you may have a leak inside booster diaphragm.