jrpisha

I am of the camp of keeping my key in my pocket, rather than inserting it in the dash to start the car. In order to start my 2015 S550, one is required to depress the brake pedal, hold it down & hit the starter button. No problem.

Traditionally, in the starting scenario above, the brake pedal will sink an inch or two under the pressure I apply to start the car. Three times this past week, under cold starting (car sitting overnight) the brake pedal would not move under the traditional pressure I would normally apply & would not start. I had to really push on the brake pedal for it to move. Once the pedal had finally moved, albeit less than the normal travel I detailed above, the car started normally. The operation of the brakes is normal, "feel" is normal, stopping distances are normal, no vibrations, no pulling, etc.

Anyone else experiencing this issue? It's cold here in Northern lower Michigan, but I keep my car in an insulated garage.

Let me know if you have any thoughts on this .

Thanks !

John

2012 merc amg

Same car I drive, 2015 S550. I have not noticed this at all. But from what you describe it sounds like you are loosing vacuum out of the brake booster maybe from a small leak. Normally the canister maintains vacuum so even after the car is shut down for quite sometime vacuum is maintained therefore making it easier to push the brake pedal. Might want to get it checked. They would apply a suction tool with a vacuum gauge to the booster to see if the gauge moves down after the suction is pumped up. If so they would find the leak and fix it. Might need a new brake booster.

Streamliner

I have a 2015 S550 as well and have experienced the exact same thing several times, but it never seems to be a problem, more of an annoyance. I live in Southern California and my car is always garaged, so it never gets that cold.

2012 merc amg

There might be a check valve on the power booster that has gone bad and not holding vacuum. Might be a good place to start.

1bad540

Think mines been doing the same thing last few days as in Ny its been 20-30... Merry Christmas!

jrpisha

Follow up. I had the car at the dealer today & they ran it through a battery of tests. No error codes anywhere. The power booster holds vacuum & the check valve is good. The dealer told me the brake booster has a sensor on it & it's not throwing error codes either. I thought maybe a lazy caliper might be causing this, but they pulled all four wheels & all the calipers are fine. These sort of "non-documented features" (problems that the dealer can't fix) are the sort of thing that led me to selling my BMW & buying this S550. Oh well, one of those things I have to learn to live with...

John

2012 merc amg

Wow, bummer. At least only around 4 months of winter left and then hopefully it won't do that once it warms up out. Did the check for any TSB's on this? Technical Service Bulletins. Seems like maybe a brake fluid flush might be a good thing to try if you have not had that done in a while, suppose to be done every couple of years on these cars, you never know maybe that would help.

1bad540

you know mine seems to be making a very hi pitch wine from around the brake pedal when im at a red light and on the brakes. comes on and off when i push the brake pedal almost like a slight air leak sounds like. If i push the pedal very hard it stops.... anyone else hear that? maybe my booster is bad?

kunzy80

Dear member
Hope you're well

Mine is Doing the same exact thing..

Did you happen to find out what the issue was

Thanks again

MBS63AMG

I wonder if the "hold" feature has anything to do with it, or even affected by this.

PNBLWZD

Sounds like vacuum is leaking down from the power brake booster or associated component, as someone stated above. I’ve had this happen on other cars when stored for weeks/months which I consider ok, but in a brief period of time like overnight it shouldn’t happen

rectified

When the engine is running it makes vacuum for the booster to help the pedal make force for less pedal effort. When the engine is not running you have no booster and no vacuum that means your pedal is going to be hard to press until you start the engine. You have no problem from what it sounds like. Best regards.