Brake failure! Advice needed pls.

[mrkozzy]

6 months ago the brake pedal went to the floor and I panicked. Couldn't see any leaks in any of the system.  Reservoir was full.  I bled the wheels thinking perhaps a little air had gotten trapped in the system.
Pedal came back hard and I thought no more of it.
Yesterday whilst traveling on a quiet country road the pedal went to the floor again.
Rolled to a stop and checked the entire system.
No leaks, reservoir is full.
Sat for 20  minutes considering my options. Started the car and brake pedal is BACK!
Drove home some 100 klms and no issue.

Master cylinder changed for a newby 3 years ago.

Has to be something wrong but where to look? As  I said there are no noticeable leaks in the system.
Might it be the brake booster finally dying a slow death perhaps. It is the original.
Any suggestions?

[Jed]

My understanding of a bad brake booster is that it will require a lot more effort to brake.  harder pedal.  Not pedal to the floor.  Very strange that it would come and go. Sounds dangerous!

My initial thought is you have a master cylinder that is failing internally. Perhaps a piston in the master is sticking then freeing up and returning to position...curious to hear what others think??

[Feather535]

It wouldn't be the booster.  A failing booster would increase braking effort; the brakes would still work, with a harder pedal.

An internal leak in the master cylinder could cause this kind of problem.  Or if you were driving fast and braking hard, it could be due to brake fade.  That would be consistent with the problem resolving after not driving for a time. 

If the brake fluid hasn't been changed in the last year, you might start there.  Over time, brake fluid absorbs moisture, which can boil and cause fading.

[daantjie]

Not sure if failing hoses would cause this but it's worth swapping out old hoses as a matter of course.  They can collapse internally and still look fine from the outside.

[JP]

Do you remember what brand is the master cylinder you replaced the old one with? I had a similar issue earlier this year and tried a handful of brands, noticed a big difference in materials and piston sturdiness between Ate, Metelli, TRW and such. Brought them to a brake shop and they bench/vacuum test them, ended up choosing the Metelli one, so far great.

Good luck to you!

[mrkozzy]

Thanks all for the guidance so far.

I'll start with a full  break fluid replace and bleed this weekend, as the last full change was about 4 years ago. (The same time the new master cylinder was installed).
Not sure of the brand of master cylinder I replaced it with at the time except to say it cane through a long time mercedes parts supplier in Australia.  I believe it may have been ATE brand but not sure.  Is the cylinder branded anywhere I wonder?  I have never looked.

Also I will take off every wheel to do a visual on the wheel cylinders. Perhaps one is sticking intermittently?

"A failing booster would make the pedal harder", Yes I agree with that so for the time being I will discount the booster.

Stay tuned

[UTn_boy]

There are no wheel cylinders on a W116 because there are no drum brakes.  You have brake calipers (disc brakes) on the front and the rear.  Even so, I'd look for signs of one of the caliper pistons sticking.  (melted dust boots on the caliper pistons, and discolored brake rotors.)  If one of the pistons is sticking it'll cause the brake to be applied constantly, which will get things so hot that the brake fluid will start to boil.  When this happens, you have what is called brake fade, and the symptom is a soft and lengthy traveling brake pedal.....but returns to normal when the brake fluid cools down. 

Otherwise, you have a master cylinder that is starting to fail.  Or, it could very well be both of the aforementioned. 

When we consider that there is no loss of fluid and good/normal braking most of the time, then that rules out any air in the system.  Renewing the fluid and re-bleeding won't fix your problem. 

Do take consideration in Daantjie's comment about the rubber brake hoses going to the calipers.  While an internally collapsed brake hose will be much more of a constant problem rather than intermittent problem, they're still suspect.  Here is why.....If they're more than 10 years old or are original, then they need to be renewed. Brake hoses are a wear/regular service item...much like a belt, hose, or a windshield seal... that requires regular replacement, and they get overlooked 99.9% of the time.  All brake hoses have date codes on them......usually just a month and year, or what week of a particular year they were made. Either way, just pay them some mind.  Even a visual look at them can sometimes tell you of they're old or not, but it's a little less reliable of a way to go about it.   

[mrkozzy]

As always, I read your post(s) with great interest UTn_boy. Thanks once again!

Showing my age when I wrote "wheel cylinders"  Obviously calipers was what was meant.

What is your thought on the front wheel (shudder) I experienced just prior to the brake pedal disappearing?  It felt like I had thrown a wheel weight.

After the restart it was normal.

Kozzy

[UTn_boy]

Normally, shuddering is from a warped brake rotor and/or an extremely worn suspension part.  The fact that it goes away is rather baffling.  When this happens again, quickly pull over and smell for burning brake pads and feel each wheel by hand to see if one is hotter than the other.  Bear in mind that under normal operating conditions the rear brakes run cooler than the front brakes, as the rear brakes only do 20% of the braking. 

[mrkozzy]

I remember stopping and lifting the bonnet.
Could smell something emanating from the engine bay, but couldn't put my finger on it.
I'm thinking we're narrowing the search down to possibly a brake (rotor?) problem maybe, or something brake related perhaps?

[UTn_boy]

You were likely smelling burning brake pads...but could have also been something engine related.  We can't really say anything about the rotors at the moment, but I suspect a sticking caliper piston.  Still, I wouldn't rule out a failing master cylinder, either. 

[Feather535]

Burning brake pads could be due to a sticking caliper.  And that could be caused by a collapsed or swollen hose or by rust or contamination in the caliper assembly.  It's also possible you overcooked your brakes by hard driving or a long, fast descent.

[raueda1]

After reading this I think a similar thing happened to me in my early days with my car.  I ended up just replacing the calipers, hoses and rotors on principal.  Anyway, along with everything else mentioned, I wonder if somehow the calipers weren't completely bled.  If they retained significant amounts of the old fluid in the calipers then I wonder if this scenario might occur:  brakes overheat, fluid boils in the caliper, vapor (which is compressible) is now in caliper and lines, hence no brakes.  Brakes cool back down, vapor is eventually reabsorbed into the fluid and brakes seem to work again. I'm pretty sure that this is what happened to me.

MB manual says to change brake fluid annually.  I got a pressurized brake fluid setup from MercedesSource and put about 2L of fluid through the system to be 100% sure it's flushed.  I calculated the volume of the system at one point and determined that this was several times more than it contains.  Cheers,

[ptashek]

There's about 500ml in the entire system. If the reservoir seal and hoses are in good shape, annual changes seem excessive.

If you don't want to keep changing the hoses every few years, buy stainless steel braided teflon ones (stahlflex is the common name) with brass or sts couplings, and you'll never need to change them again.

Something like this: