Should a brake booster hold vacuum? - PeachParts Mercedes-Benz Forum

offalot · #1 09-12-2019, 04:35 PM

While trying to trace down a vacuum leak because the engine won't shut off, tested about every little vacuum path then lastly, disconnected the brake booster. If I put the mityvac on the hose connecting everything else it holds a vacuum oh, but when I turn it on the booster it doesn't pull anything. This is a new-build and while test-driving it the brake pedal was definitely hard. Does this mean that my booster was bad? Are there any other ways to test it?

pawoSD · #2 09-12-2019, 05:12 PM

I think in the resting state it should hold vacuum.....its only vented/used when applying the brakes and releasing.....if it leaked all the vacuum away it would not provide any assist. Based on the hard pedal, probably a bad booster.

offalot · #3 09-12-2019, 05:26 PM

Quick update. As I said, it was holding a good vacuum when I put the money back on the hose that was disconnected from the booster. Well I forgot there is a check valve there. I found a few leaks and repaired them and with a cap on the hose that normally goes to the booster the engine shut off with the key. But then when I attach the hose back to the booster the engine does not shut off and there is no brake feel. So yes, I'm guessing a bad booster.

Diesel911 · #4 09-12-2019, 06:40 PM

Quote:

Originally Posted by offalot

Quick update. As I said, it was holding a good vacuum when I put the money back on the hose that was disconnected from the booster. Well I forgot there is a check valve there. I found a few leaks and repaired them and with a cap on the hose that normally goes to the booster the engine shut off with the key. But then when I attach the hose back to the booster the engine does not shut off and there is no brake feel. So yes, I'm guessing a bad booster.

Before you condemn the Brake Booster there is an O-ring between the Master Cylinder and the Brake Booster that could cause a leak.
That is the way it is on my year and model.

123boy · #5 09-12-2019, 06:55 PM

Definitely 2 thumbs up for what Diesel911 is saying. Same thing happened to me. Was gonna yank my booster, but glad I didn't. Avoided a huge headache for a 2 dollar o-ring!!
Brake booster: Small mechanical victory, HUGE psychological victory
Good luck!
P.S. I got mine from Mercedes Source. I had it in a couple days......not difficult to swap out. Inquire here, if not sure.

offalot · #6 09-13-2019, 01:50 PM

Great. I will definitely check that out. Would you just suggest taking off the master cylinder and doing a visual inspection, or some kind of pressure, vacuum test? After the sticker shock of a new brake booster, I am definitely crossing my fingers for the o-ring.

barry12345 · #7 09-13-2019, 02:16 PM

Used one is usually good. If that is what it is.

123boy · #8 09-13-2019, 04:38 PM

No need to remove the master, just crack loose the flare nuts that hold the brake lines to the cylinder. Before starting, wipe the face of the booster and around where the cylinder bolts up. Don't want any dirt or debris to get into either part when they're separated. Be sure and only CRACK the flare nuts. They shouldn't be so loose that brake fluid drips out....put rags down, just in case, or don't, if you're not worried about it (do this BEFORE you unbolt the master from the booster). This will allow the unit to pivot on the brake lines as you pull it forward. After you unbolt the master cylinder from the booster, it only needs to move forward an inch, or so, to finagle the o-ring in place (remove old one first, of course. mine was completely missing). Before you do this, make sure both mating surfaces are clean and dry. I like to apply a small amount of lubrication to the o-ring to prevent it from drying out and keep it pliable, over time. You'll have to get the o-ring around the back of the master cylinder and in it's groove (USE CARE TO NOT DROP THE O RING INTO THE BOOSTER). After its seated, bolt everything back together, tighten the flare nuts, pump up your brakes (to make sure you don't need to bleed them), check for leaks at the flare nuts, and........start the car and see how your pedal pressure feels.
Helpful hints: Get yourself a flare nut or "line" wrench, if you don't already have one. 11mm is the nut size. They look like a double ended box wrench (aka "dog bone"), so there will be 2 sizes in one (10mm and 11mm, or, 11mm and 12mm). They're not that expensive and an invaluable tool for any type of brake work. Open end wrenches will only round out your flare nuts.....then, you're hosed.
DO NOT TOUCH the brake pedal with the loosened master cylinder flare nuts! you'll suck air into the system, then brake bleeding WILL be required.
As always, additional help (and plenty of other opinions) can be found "here". GOOD LUCK!!

Assault · #9 09-13-2019, 05:54 PM

An obvious sign of a bad seal that is being discussed is a rust stain or flaking paint directly under the BMC, which is also a sign that the BMC is on its last leg.

123boy · #10 09-13-2019, 06:13 PM

Assume nothing. An "obvious sign" can turn around and bite you in the ass....lol. Many boosters have "flaking paint" due to brake fluid being spilled or leaked onto them, over the years. Brake fluid is a GREAT paint stripper/flaker. The "seal" in question, has nothing to do with brake fluid retention or rust. It's an air seal.
The booster is either shot or it is not. Personally, I'd try the $2 o-ring before r/r of the brake booster. Keep in mind, the booster cannot be tested for vacuum integrity with a bad/missing o-ring. Gotta check/change that to confirm a bad diaphragm. If the booster is bad/unserviceable, you'd change the o-ring anyway, so now you already have that part and have eliminated it as the "cause" of your problem.
The brake master cylinder is another issue, entirely. The original post concerns a "vacuum leak". Unrelated to brake hydraulics.

Assault · #11 09-13-2019, 06:33 PM

Very clear sign that the seal and BMC has failed. Note the line of rust:
https://mega.nz/#!9IsgDCAC!wVYaYJMH6Mrs35ykpgODMFkv_03vabnuYjZ_1QHN9Ww

Of course replaceing the 2$ seal first before replacing the BMC or doing any serious break work is the best option if it fixes the problem. I have yet to experience (fortunately) a booster failure.

Now in my particular case it was due to failed BMC internal seals causing break fluid to break the seal between the BMC and the booster. I also had to replace the check valve going to the booster as it was rusted out inside and rendered useless.

Does the car shut off normally while the break booster is blocked off from the vacuum system and the vac line sealed off? Have you tried capping all the other vac lines and only running vac to the booster?

If the BMC is original it could have failed causing hard breaks. OP has not stated if the car has been sitting and if so for how long. If it has been sitting for a long time and the BMC is original it could be a combination of frozen calipers pistons and or BMC that has failed.

123boy · #12 09-13-2019, 06:52 PM

Yes yes! Good points, all. Nice link. True, there is much missing info with respect to this post/issue. Wouldn't want to wish a BMC replacement on my worst enemy.....lol (kidding, not really that bad)