W126 spongy brake pedal

[sailor]

Hi everyone

Have just completed registration, this is the first time I have used the forum (any forum come to think of it) and would value your advice on a problem relating to my 1989 500SEL.

A couple of days ago whilst movinging in typical town traffic I was caught unawares (panic) when my brake pedal travelled almost to the floor when applied before the brakes operated.

At the first opportunity I pulled into a side road and parked up with the engine running.

I tested the pedal again, excessive travel still present and a definate whosshing sound accompanied pedal depression.

Tried turning off engine for a couple of minutes and lifted bonnet to check fluid level which was OK.
Restarted engine but no change so drove back home carefully.

Restarted car an hour later, pedal felt fine (minimum play, hard when operated) completed short trip around the block, brakes perfoming OK.

My initial diagnosis was a 'brake servo' fault which corrected itself.
Grateful for any help please.

 

[television]

OK the first thing is to check if there is any staining or wet looking patches on the brake back plates and wheel, if ther is it means that a wheel cylinder/ calerper is leaking, though I do not think it is the case.

The brake master cylinder is a tandem device, that means one piston for the front and one for the back. all on the same piston rod. where the master cylinder bolts onto the servo check for any leaks or oily stains here, it would have stripped off any paint on the face of the servo if it was leaking.

If all OK there it means that one of the piston seals has gone, the hissing you can here is the action of the servo. The servo has nothing to do with this fault. If you find that the pedal slowly sinks again it will need a new master cylinder, easy to fit but you will have to blead the system.
for now, if it sinks when you are driving ,you can pump it up with the brake pedal for now.

Malcolm

 

[bitonw]

If all OK there it means that one of the piston seals has gone, the hissing you can here is the action of the servo. The servo has nothing to do with this fault. If you find that the pedal slowly sinks again it will need a new master cylinder, easy to fit but you will have to blead the system.
for now, if it sinks when you are driving ,you can pump it up with the brake pedal for now.

Malcolm

Thanks Malcom,

This sounds a bit what I have with my C220 CDI (W203 2003). It doesn't always like that and some times (couple days) the brakes are working fine... What I do indeed is pump it up when it happened. According Mercedes Benz Direct Birmingham and Mercedes Benz Birmingham Central there is nothing wrong with the brakes...

bt

 

[television]

Thanks Malcom,

This sounds a bit what I have with my C220 CDI (W203 2003). It doesn't always like that and some times (couple days) the brakes are working fine... What I do indeed is pump it up when it happened. According Mercedes Benz Direct Birmingham and Mercedes Benz Birmingham Central there is nothing wrong with the brakes...

bt

Just done my Volvo, it was like it for months but started getting very bad, as you say it does not always show up. though they all react differently I could make mine do it by keeping a slight presure on the pedal it would slowly sink.


malcolm

The effect of pumping it up forces the seal open for a bit, then after it reverts back to its gone shape

 

[bitonw]

Excellent info!

But I have still warranty so they have to do the job and stop telling me "There is nothing wrong with the brakes sirr, Mercedes brakes are always soft". At the moment I have contact with Maastricht (NL) about my complain of not sorting this problem.

 

[sailor]

Malcolm

Over the next few days I will check for 'fluid leaks' in the areas you have suggested.

 

[television]

The reason why the rubber seals go in the master cylinder is that the front to back ratio is 72% front and 28% back, the rear washer can go to sleap.

Malcolm

 

[bitonw]

Went to Mercedes Benz Coventry after my work in the afternoon to have a chat with the Service Manager. He did do this test and told me that there is a problem with the brakes and that I should book the vehicle in.

Auto box on park and foot brake applied. With his right foot on the accelerator to keep the engine on about 1500 rpm. Than with his left feet on the brake pedal and the the rpms went almost down to about 500 rpm.

What is this test telling me?

 

[television]

Went to Mercedes Benz Coventry after my work in the afternoon to have a chat with the Service Manager. He did do this test and told me that there is a problem with the brakes and that I should book the vehicle in.

Auto box on park and foot brake applied. With his right foot on the accelerator to keep the engine on about 1500 rpm. Than with his left feet on the brake pedal and the the rpms went almost down to about 500 rpm.

What is this test telling me?

I think he was doing what I was saying, keep a light presure on the pedal and it slowly sinks, the engine is only running to fill the servo.

Malcolm

 

[bitonw]

Ok, But why did the rpm dropped from about 1500 rpm to 500 rpm while he keept his right feet on the accelerator? The moment he removed his left feet of the brake pedal the engine when back to that 1500 rpm. What was draging the engine down? Was it a vacuum leak what caused the drop in rpm?

 

[television]

Ok, But why did the rpm dropped from about 1500 rpm to 500 rpm while he keept his right feet on the accelerator? The moment he removed his left feet of the brake pedal the engine when back to that 1500 rpm. What was draging the engine down? Was it a vacuum leak what caused the drop in rpm?

.

The brake servo has nothing to do with the running of the engine.

Malcolm

 

[bitonw]

Ok, thanks.

Vehicle is booked in in 2 weeks so I will see and let know what they did.

bt

 

[sailor]

W126 spongy brake pedal situation update

Hi everyone

After following up on sound advice from from senior members I have carried out checks as advised but am unable to detect any brake fluid leaks.

The spongy pedal syndrome appears to have spontaneously resolved itself and is now back to its previous operating hardness.

Thanks again
sailor

 

[bitonw]

Hmm, May be an air pocket moving around in you brake pipes?

 

[Myros]

Sailor, re your 1st post

I bet it was a momentary overheat/fade caused by a binding brake, which freed itself when you pumped hard. The offending caliper will have boiled the fluid, giving you a vapour pocket and the pedal down to the floor effect. Once the vapour cooled, it would return to its liquid state, the offending brake having freed itself, would no longer overheat and cause another fade.

It's worth having all the pistons looked at and seeing if any are sluggish in returning on relief of pedal pressure. The next time this happens you might not be as lucky or have as much room to recover.

 

[bitonw]

No, it's as Television already told us the Master Cylinder! Mine will be replaced on Oct 9th under warranty by Mercedes Coventry.

For some days my brakes are fine and than suddenly the brake pedal sinks more down than normal. I just pump a couple time so I get better braking. But for now I just gone wait till they change the Master Cylinder.

 

[IAN MAC]

Heres a thing, i play with stock cars for a hobby and i had the same problem,( nearly ). Brakes would suddenly go to the floor, not a good thing when your racing, left for 5 minutes they came back to normal. changed the master cylinder and found it all gummed up with a white substance, dont know what it was but no problem since.

So looks like your on the right road.

 

[lepton]

Same problem with my 126. Replacing the master cylinder cured the fault completely. The brake fluid which hadn't been renewed for some time was pretty murky.

 

[MW]

Brakes are not animals that have a bad day. If a fault develops, it is ALWAYS a fault. Spongy peddles that 'get better' are almost always the master cylinder seals.
In the old days with just one simple system, if it failed, the peddle went to the floor as there was no back-up system or the additional complexity that clouded diagnosis. When it failed, it failed.
If the wheel cylinders (or calipers) are OK, then the fault can only be weak hoses, or internal master problems.

 

[television]

Brakes are not animals that have a bad day. If a fault develops, it is ALWAYS a fault. Spongy peddles that 'get better' are almost always the master cylinder seals.
In the old days with just one simple system, if it failed, the peddle went to the floor as there was no back-up system or the additional complexity that clouded diagnosis. When it failed, it failed.
If the wheel cylinders (or calipers) are OK, then the fault can only be weak hoses, or internal master problems.

The other thing to throw people off the scent is that modern systems use a tandem master cylinder. With these the pedal does not go down to the floor as one half is still working.

malcolm