Losing brake pressure at times
 

My 86 109e 16v has been losing pedal pressure at times. Normally the brakes are firm, but occaisionally in traffic they go almost all the way to the floor. It only happens when I am holding the clutch in at the time (stop and go traffic mostly). It feels like only the front brakes are grabbing when this happens as the car nose dives hard. It doesn't happen all the time though, so far twice in the last week. As soon as i get moving fast again, the brakes are fine.

Any ideas?

sounds like it is time for a new master cylinder. it sounds like an internal leak.

tom w

I second tom w. This usually happens when you are light on the brake pedal...Right? When you push the brakes moderate to hard the car stops ok....Right? If this is so, what is happening is when you push hard the master cyl cups expand to seal in the m. cyl. bore. when you apply light pedal, the cups will let fluid leak past.

It happens when I am light on the brake and the clutch is depressed (Stop and go traffic on, but not everytime)

I think that it has nothing to do with the clutch being pushed in. Replace the master cylinder. Be sure to "Bench bleed" the master cyl. It will save you alot of time on the system bleed.

Based on your description of the symptoms, I wouldn't dismiss the clutch connection so quickly.

I was surprised to find on my 300E that the clutch and brake share the same fluid reservoir, although with typical German efficiency the takeoff for the clutch is higher than the brake so if your clutch hydraulics spring a leak and drain dry, there's still enough fluid left in the reservoir to maintain the braking system. I know this for a fact because I drove 30km downhill without a clutch recently after a master cylinder failed.

Also, am I right in thinking that the independent brake systems are split front back, not diagonally? There's a clue there somewhere.

And, I know these are probably stupid questions, but have you:

checked the fluid level?
looked for evidence of leaks on the garage floor?
changed fluid recently?

You don't say how long you've had the car but it's old enough to seriously consider overhauling all your hydraulic systems. One of the first things I did when I bought my car was change all the rubber brake lines and change the fluid. Its worthwhile servicing the master cylinders as well because the seals will give up eventually, and they start leaking when hot. New units aren't expensive but you can also strip them down and replace the seals yourself if you're that way inclined.

A 20 year old car, you should probably think about replacing all the fuel and water hoses as well while you're at it. save yourself a lot of grief later on.

good luck

the brakes are connected front to front and back to back. not diagonally.

tom w

If it were me, the FIRST time this happened would be the LAST time. I would pull over and have the car towed, and then not drive it until it was fixed.

If you continue to drive this vehicle before getting it fixed, you are endangering yourself and everyone around you.

You are also taking a huge liability legally, driving a car with a known brake problem.

if i were a lawyer that would be my advice.

tom w

To answer the questions:

Fluid level is full
No leaks on garage floor
Fluid is dirty though and will be replaced next weekend.

Only had this car 3 weeks now. I have new rear rotors, pads, e-bakes and cables to install along with Braided steel brake lines. Going to flush all the fluid when I do this, then the fronts next.

I have a spare master for it, just wanting to avoid having to bleed the clutch if I swap masters cylinders.

Thanks!

And when I lose pressure it seems the fronts are the ones left as the car nose dives hard....Normal braking is real level (or at least it has been for me) in this car

I'm not driving it now, its parked until I can figure it out. We'll be moving soon as well and its one of the cars I will have shipped.

jskibo, Other than to "change" the clutch hyd fluid, you should not have to worry about loosing clutch pressure with the master resevoir removed. just don't push the clutch in while the resevoir is off. Bleeding a clutch is easy.

I don't know how you are going to ship your car but if anyone besides you are going to operate it atleast tell them or atleast change the master cyl. My 2 cents..... Chris

i agree with motorhead except bout the bleeding of the clutch cyl. it can be a bit*h!

tom w

Bleeding of the clutch is a three man job. One on the pedal, one under the car holding the hose on and spannering the bleed screw and one keeping the fluid level topped up in the reservoir...

I believe there are power bleeders that may help but this is the old school way of doing things.