W124 E300 diesel hard brake pedal

[thomas forsyth]

I have had absolutely no success in establishing the cause of the intermittent fault in the braking system of my car which led to a serious accident earlier this year, can any member(s) throw light on this for me. In December 2001, I and my wife came down a slip road off the motorway to approach a set of traffic lights at red, applied my brakes, brake pedal rock hard and would not depress, went through the lights at red,and with good luck no one was coming from right. When car did come to a halt, tried the brakes to find them working as normal. The diagnosis of the two motor engineering companies I use (Scotland and England) was that the vacuum pump to servo had failed, I had this replaced immediately. April 2003, driving downhill towards traffic ahead, braked to fall into line, and the same effect recurred, rock solid brake pedal, was unable to stop with disastrous results, fortunately no one was injured athough cars left the road. Traffic Police attending the scene took my car for a drive, and as happened previously, the brakes were operating as normal, the car was taken by the DTI for testing, again, found no fault. I am at my wits end to have someone identify or advise me as to the likely cause of this fault, has anyone experienced this. In the meantime, my lovely car has been parked up on the driveway since April being unsafe to drive not knowing when this would recur.

 

[dieselman]

Really you need to establish that you have continuous vacuum for the servo, then you need to know that the vacuum is applied as you press the pedal. A vacuum guage should reveal these situations.

There should be an air filter on the pedal rod side of the servo which allows air into the back of the servo diapragm when the pedal is pressed. If this filter is blocked there will be no air pressure in the back of the servo so no additional pressure to act on the diapragm.

Really the servo needs overhauling or exchanging, assuming you have a guarenteed vacuum from the pump.

 

[thomas forsyth]

Thank you so much diesel man for the prompt reply to my question, however, you could apply a vacuum guage to the system at any time and get a positive reading, the problem is the intermittent nature of the fault i.e. dec. 2001 - march 2003, I really need to know which component( i.e. valve or as you suggest air filter) in the system could possibly cause this brake failure. I have found the Mercedes dealers in Scotland of absolutely no assistance in the past 8 months, nor are they even remotely interested,
one of them wrote to Mercedes headquarters in Milton Keynes and did not receive any acknowledgement. By the way, the car is 1993, mileage app. 235,000 fault free miles.

 

[Andy]

Hi

I agree with Dieselman. I would go a step further ESPECIALLY when you had the problems you have.

In my opinion the main focus of your efforts would be

1.Change the vacuum pump (Which you have already done)

2.Replace the servo with a new one (not second hand) @235k you had your money's worth. Also change the vacuum pipe from the servo to the vacuum pump as Diesel man has suggested.

This should cure the problem. I doubt the brake master cylinder could cause this problem. Its unlikely that it would cause the brake pedal to be rock solid

While changing the servo check the clevis pin & all mechanicals from the brake pedal bracket for wear and proper operation.


Regards

Andy@ www.mercedesservicing.com