Brake Issue - No Warning Lights - CLS 2017

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MCW83

New Member
Joined
Jun 21, 2021
Messages
6
Location
Wirral
Car
Mercedes CLS
Hi Everyone,

During a really short drive on Friday night, I went to apply the brakes, but felt major resistance....and had to force my foot down to slam on in order to stop. The car stopped, and then oddly when I started driving again the brakes were absolutely fine. There was / still is no warning light on the dash. I've driven the car a couple of times over the weekend and all seems ok. Took it to a local mechanic to look at quickly today....pads, brake fluid all ok - no clear sign of an issue.

I'm meant to be doing a long motorway drive tomorrow - but am pretty nervous about it now!

Has anyone encountered similar to this before? Or any idea what it might be / how safe it is to drive still!

Many thanks!
 
Is the floor mat securely located? No possibility it or something may have slipped under the brake pedal?
Did the mechanic run a diagnostic scan to check for error codes?
 
Is the floor mat securely located? No possibility it or something may have slipped under the brake pedal?
Did the mechanic run a diagnostic scan to check for error codes?
Yeah the mat seemed ok, and I did a quick check to see if a bottle or anything might have rolled underneath it.

Unfortunately the mechanic didn't - I couldnt find anyone who could have a proper look at the car today (other than paying Mercedes main dealer £175 + VAT to look at it).
 
Is your life ( or someone else’s ) not worth £175 + vat?
 
Is your life ( or someone else’s ) not worth £175 + vat?
Absolutely - I'm just trying to explore other options first before having to resort to the main dealer.
 
Absent any obvious interference with the brake pedal (floor mat or other foreign object) it sounds like a potential issue with the brake servo. Was it accompanied by any odd engine characteristics such as an unstable idle or unusual idle speed?

It sounds as though the servo is working ok again, but if you want to do a static check proceed as follows. With the engine stopped, pump the brake pedal a few times until it goes very firm when all the residual vacuum is exhausted. Then, with your foot pressing firmly on the brake pedal start the engine and you should feel the pedal drop as vacuum builds in the servo.
 
Absent any obvious interference with the brake pedal (floor mat or other foreign object) it sounds like a potential issue with the brake servo. Was it accompanied by any odd engine characteristics such as an unstable idle or unusual idle speed?

It sounds as though the servo is working ok again, but if you want to do a static check proceed as follows. With the engine stopped, pump the brake pedal a few times until it goes very firm when all the residual vacuum is exhausted. Then, with your foot pressing firmly on the brake pedal start the engine and you should feel the pedal drop as vacuum builds in the servo.
Thanks @st13phil - much appreciated. The mechanic had suggested that it might have been the servo - by the time I'd gotten home I couldn't for the life of me remember what he'd called it!

Not sure if this is something you'd know, but is it best to replace the servo now? Or is it generally ok once it's essentially reset itself?
 
Brake servos tend to hard fail rather than work intermittently. The reason I asked about any odd engine characteristics accompanying the issue is that the most likely cause of a problem would be a vacuum leak between the engine and the servo, or alternatively a failure of the vacuum non-return valve.

Personally I wouldn’t just change a brake servo on the off chance that it might be faulty as they are generally very reliable so you may still end up with the same fault. You need to get someone to properly examine and test the vacuum hose(s), non-return valve and servo rather than just stabbing in the dark.

I know that's not particularly helpful if you have a long motorway drive tomorrow, but it’s the only way to be certain.
 
Brake servos tend to hard fail rather than work intermittently. The reason I asked about any odd engine characteristics accompanying the issue is that the most likely cause of a problem would be a vacuum leak between the engine and the servo, or alternatively a failure of the vacuum non-return valve.

Personally I wouldn’t just change a brake servo on the off chance that it might be faulty as they are generally very reliable so you may still end up with the same fault. You need to get someone to properly examine and test the vacuum hose(s), non-return valve and servo rather than just stabbing in the dark.

I know that's not particularly helpful if you have a long motorway drive tomorrow, but it’s the only way to be certain.
@st13phil that's really helpful - thanks. I'll be taking the train then and getting it booked in somewhere.
 
I'll be taking the train then and getting it booked in somewhere.
A wise decision, IMO.

Please let us know what transpires when it’s been checked over.
 
@st13phil FYI - been in to a MB specialist today. They cant find a fault, having checked all the vacuums / pressures etc. If not servo - suggestion that it might have been an engine issue - perhaps stalling momentarily or something. Not sure whether it's a good thing or not that they didnt find anything! Would rather they'd seen something obvious that could reassure me!
 
Thanks for coming back and updating the thread.

Hopefully it was a one-off glitch that doesn't repeat, but it's never reassuring when something like this happens and you receive a "no fault found" diagnosis.
 
I've NEVER liked self curing faults ! the little sods have a habit of coming back, it HAS to be Star for your peace of mind.
 
'Self curing' faults are the worst and I for one would not be happy with one in the braking system, but the OP has had it checked by a specialist so its difficult to say where to go from here.
 
Star !
 
This is exactly whats happening to me and just done the diagnostics and it showed nothing. No lights on dash either. Really worried to drive my pride and joy :-(
 
'Self curing' faults are the worst and I for one would not be happy with one in the braking system, but the OP has had it checked by a specialist so its difficult to say where to go from here.
Surely there would be a stored fault, even if the original 'fault' had rectified itself?
STAR should show all current and stored faults, unless of course the operator cleared them before checking,,
 
Thank you Whitenemesis for sending me this thread but the mystery continues. I'll post back when I get to the bottom of this. Until then... I'll be staying off the road :-(
 

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