[W126 Coupe] Brake Master Cylinder

[Dick Spellman]

Hi Sam,

I am a proponent of flushing all the sludge and moisture laden brake 
fluid on the lines using a pressure bleeder set to 30-35 lbs every two 
years.  If this has never been done, I recommend you replace both rubber 
pressure caps on the brake fluid reservoir first. After 10 years they 
crack and doing a pressure bleed will let you know this in short order 
and makes quite a mess.

I should also mention that at or about 15 years, in my limited 
experience the rubber brake lines to the calipers tend to start swelling 
from the inside causing undo braking action once things heat up.  So, if 
in doubt, rip these out and replace while you are doing the system.

As for the master cylinder, it's a straight forward replacement.  A 
couple of comments.  Once the old one is removed, inspect inside and 
down for any signs of build-up with a flashlight.  If the build up has 
risen to where it's in contact with the booster diaphragm you may need 
to replace the booster sooner rather than later.  Assuming the build-up 
has not made it to the rubber diaphragm, just slide the master cylinder 
in directly with no angle to get it to seat without damage.

Take care,

Dick

samsisic at comcast.net wrote:

> Hi all,

> Just wondering if there is anything special about replacing the Brake 

> Master Cylinder...

> 86K miles, 20 years... Time to replace? Not getting good pedal feel. 

> Pedal will go to bottom with applied pressure. Could be air in system 

> or bad Master Cylinder? There is no leaks in the system.

> It looks like two bolts at the brake booster and two brake lines need 

> to be disconnected, pull out with fluid reservoir still in place? Rags 

> under assembly to catch any brake fluid...

> Pour out old fluid from reservoir, install new Master Cylinder and 

> reservoir in reverse and bleed system with new DOT4 fluid?

> Any advice is appreciated.

> Thank you,

> Sam

> Chicago

> 88' 560 SEC US

>  

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