W123 Braking: Sudden loss of pedal pressure boost

[Screwdriver]

I was driving my W123 300d (non-ABS) on the A13 today and after about an hour of driving at 65+ mph, I experienced a sudden loss of brake assist/ boost. Here is all the information I have:

• The pedal depressed much further than usual repeatedly before any braking took place

• Pumping repeatedly brought back most of the assist/ boost

• Brake fluid did not appear to be leaking. The reservoir was near full and I could not spot any wetness in the wheel well.

While the car appears to brake normally now, I'm concerned and want to know what the probably causes are. I'm guessing it is air in the system but wanted a more experienced diagnosis. Many thanks in advance!

 

[SLKNick]

Air doesn't suddenly just get into the system, more likely you managed to boil the brake fluid. When was the last time it had a proper fluid change?

 

[jaymanek]

Master cylinder most likely.. Could be a brake flexi pipe too

 

[a111r]

Mine boiled the brake fluid due to a seized front calliper piston.

 

[Screwdriver]

Recently replaced the rear brake metal pipe following which the fluid was replaced. Sounds like it could be a couple possibilities. I drove the car today and didn't experience any loss of pressure.

Any suggestions on next steps?

 

[bolide]

Servo on those cars is provided by a vacuum pump and, IIRC, a vacuum reservoir. If the system is working OK now I'd be looking at those

Nick Froome

 

[Screwdriver]

Cheers Nick!

 

[Screwdriver]

Update: I've driven the car for 150 miles or so since this incident occurred and the brakes appear to be fine. No loss of fluid or braking pressure. I'm even more confused now and am concerned on how to ensure this doesn't happen again.

Any advice would be very appreciated!

 

[grober]

My guess faulty Master Cylinder and/ or Brake Servo as Jay said in post #3 . Because you don't want to take a chance with brakes might be worth investing in a professionally rebuilt unit. This lot seem to know what they are doing??
Brake Parts | BrakeParts.co.uk

 

[Screwdriver]

Money isn't the issue and I'd happily purchase a new one from MB. The question I have is: If the Master Cylinder is on its way out, shouldn't the symptoms have returned?

 

[notsofast]

The symptoms sound a little like a caliper piston crept back in and needed pumped out. This is not common on fixed car discs so is fairly unlikely. If you have a warped disc it can push the caliper piston back in more than normal but you would feel the warp under braking anyway. Also if the pads are seized on the caliper sliding surface then this can cause similar issues. More common on the rear and easily identified by a poor looking disc surface.
Failure of the booster system leaves a hard pedal rather than excessive travel that needs the brake pedal pumped.
I did have a leaking master cylinder on my old W124. The only symptom was with the car stationary it was possible over a few minutes to slowly depress the brake pedal well past the normal brakes locked position by very lightly but continuously depressing the pedal. I obtained a seal kit but there was pitting to the master cylinder bore so I replaced the whole deal (corrosion due to not changing the fluid and the car left idle for a couple of years).

 

[Screwdriver]

Quick follow up:

I took a look at my brake fluid and sure enough, it is now brown in color. The brake fluid was completely replaced 6 months ago, so I'm guessing the Master Cylinder seals have given way. Any other thoughts?

If not, looks like I can

1) Rebuild the BMC with a new seal kit
2) Replace the Bendix unit with a new ATE unit.

Appreciate the advice in advance!

 

[a111r]

Servo diaphragm contamination?

1. Masters, less so slaves, can be 'tricky', assessing the wear, OK / Not OK. (not done an MB one).
2. Yes, if compatible?