soft brake

[poseidon]

Hi,

I noticed my brake pedal is getting softer. If I pump the brake seeral times it stiffens and brakes better.

There is no obvious leak anywhere, nor does it consume brake fluid. I bled the system thoroughly, few days ago but this did not improve the situtation. My diagnosis (and fear) is I need a new master cylinder. How could I be sure? If a seal is leaking on one of the wheel cylinders would I get similar symptoms?
Which one is it?

A new master cylinder is around GBP150, and I plan to replace it myself.
I did not expect a failed master cylinder at 80k. I never had similar problem on other cars. I am getting into a stage where I am fitting a new component on my car every month!!!

Thanks in advance for replies.
Regards,

Pose.

 

[television]

This can happen, I had it at 40k with my V70R its just one of those things.

 

[wireman]

Try changing the brake fluid, it will give you an oportunity to bleed the brakes correctly.
It is usualy age in years not miles that take the toll out of hydraulic braking systems.
Follow the makers bleeding proceedure at least in as far as which wheel to bleed in what order. If the correct sequence (and engine running) is not followed you can end up with a tiny quantity of air trapped in the space betwixt the two working halves of the tandem master cylinder this results in a somewhat intermittently soggy feel to otherwise healthy brakes.

It is very sensible to change the fluid at one year intervals, in my 124 owners book it sugests every year after winter is over. When I change fluid I also make sure that the calipers get fresh stuff in them by pushing the pistons back to empty them into my bleed bottle. Blackish used fluid may indicate seal degradation.

Not changing the fluid ever may lead to enlargment and softening of the rubber seals in all the braking systems components potentialy endangering life and limb, this is less likely to happen in modern dual circuit brakes but it can and I have seen one where it did and made a big big mess. The fluid is hygroscopic and will become water contaminated just for being in the cars system.

Being 10 years old your master cylinder seals aging/failing is a prime suspect for the behaviour you describe.

I would not hesitate to bypass MB spares and get one from ECP or GSF if their price were better, it will most likely be from the same maker as the MB unit.

 

[stats007]

Think yourself lucky - I just had a new one on the Porsche - £740!

 

[8~)]

Second the proposal to change the fluid - it'll tell you what you need to know. If it is the MC they're not difficult to change.

Edit - did you check carefully the flexi hoses? Had a w123 that had symtoms you describe and the culprit was a hose that appeared to be letting in air... it was worn but not leaking fluid. I changed it, bled the brakes and all was well again... it could have been I bled the brakes well but I suspect the hose was letting air in but not letting fluid out.

 

[andy03071981]

Hi,

I noticed my brake pedal is getting softer. If I pump the brake seeral times it stiffens and brakes better.

There is no obvious leak anywhere, nor does it consume brake fluid. I bled the system thoroughly, few days ago but this did not improve the situtation. My diagnosis (and fear) is I need a new master cylinder. How could I be sure? If a seal is leaking on one of the wheel cylinders would I get similar symptoms?
Which one is it?

A new master cylinder is around GBP150, and I plan to replace it myself.
I did not expect a failed master cylinder at 80k. I never had similar problem on other cars. I am getting into a stage where I am fitting a new component on my car every month!!!

Thanks in advance for replies.
Regards,

Pose.

I have the same sort of problem and changing the master cylinder and doing a fluid change made no difference, see link below!

http://forums.mercedesclub.org.uk/showthread.php?t=30450

 

[wireman]

If you repeat the bleeding process does it squirt out any air and go all better again?

 

[wireman]

My reason for asking this is-

If the master cylinder is not fully returning to its correct rest position the fluid channel that tops up the system will not fully open and a partial vacuum will be set up in the whole braking system, normaly the channel will allow fluid to fill the tiny tiny space that will always occur due to the pads wearing and the caliper pistons returning to a position ever so slightly advanced for their rest position before the brakes were applied. The partial vac will try to draw air into the system via the caliper seals or any other bit of plumbing which is designed only for positive pressure, this could be the cause of mysterious extra travel or air collection in the brake hydraulics.

This is not in my experience common I have seen it on a mini with a rusty pedal box that prevented full master cyl return and on a cortina with a faulty plasitc brake light switch that whilst it worked as a switch was decomposing and did not allow the pedal and hence the m'cyl to return to its proper rest position. I am only talking about the last millimetre or so of travel here so its unlikely that a foot is going to feel the loss of return travel.

In our more modern cars there is an inline servo booster betwixt the pedal arm and m'cyl this might also be a potential cause of reduced return space, the control valve (which regulates the differential pressures in the two sides of the servo diaphragm casing) within the servo has several parts in line with its actuating rod ("reaction disks") some of which are made of plastic or rubber compounds, one of these could at least in my minds eye be faulty leading to the postulated lack of m'cyl return. The control valve is mounted in the middle of the vacuum diaphragm and is only accesible if the servo can be disassembled, this is not to recomended as any basis of a repair proceedure, there is a jack in a box spring in the servo and its dam'n near imposible to reassemble them.

In addition to the valve and reaction disk there is a concentric air filter that is supposed to stop bits geting into the vacuum system of the booster itself, if this filter gets choked poor servo performance can result.

I have only once experienced a broken up reaction disk, this was on a 1500 alegro and the symptoms were very harsh braking with almost no possiblity of stopping gently, the disk was well and truly flattened and had no give whatsoever.
It is not outside the limits of my imagination that a similar but opposite effect could be present in other servo booster designs they all work the same way its just that different designers use different methods to make better or cheaper servo boosters.

 

[wireman]

Look at the other recent postings about brakes, it seems that there is a travel adjusment on the master cyl and pedal arrangement.

 

[television]

Thank you for the 2 post above, very informative

 

[grpar]

Poseidon

My C180 exhibits exactly the same trait as yours, but thankfully when you hit the pedal hard it always has plenty of dig. Be interested to know if MB consider a soft pedal as a problem.