w126 brake master cylinder replacement

[john rh]

Hi,

I'm still really enjoying my 500 SEL. I've done 30,000 miles since I bought it in 2010 and it is going well.

However, I'm getting slight brake drag now, and the front brakes are slow to release sometimes, despite the car having new flexy pipes, callipers and pads (which move freely).

I am going to replace the master cylinder. Can anyone link me to a good how-to guide? And/or, any particular points I should look out for doing that job?

Thanks

John

 

[jaymanek]

To be honest its a fairly simple job for a home mechanic on a W126..

- suck out brake fluid from reserviour
- cover entire area under master cylinder in rags etc
- carefully pull off the reservoir
- undo the unions, and the bolts holding the master cylinder
- replace
- refit bottle
- fill with DOT3 or 4 fluid
- bleed.. I prefer pressure bleeding and start with rear left brake, then rear right, then front left then front right.

If I were you id take the chance to replace the brake flexi's too.. people overlook these and they can cause all sorts of issues.

 

[john rh]

Right, thanks.

Yeah, I have already replaced the fronts; I am replacing rear callipers, flexis, discs and pads as part of this job.

Cheers

John

 

[ivan1983]

I replaced the master cylinder on my W126 about a year back.

Its a very easy intuitive job as Jay says, the only thing I did which was a little tricky was to pre bleed the master cylinder. Not sure if it was necessary but i did it anyway.

Basic procedure was something along these lines

Bench Bleeding the Master Cylinder - Daniel Wong

I had someone to help me bleed the system using the pedal method starting from the caliper furthest from the MC working my way to the MC.

 

[john rh]

That's useful.

I have a pressure bleeder, so I think I'll try without the prior bench bleeding. (If that doesn't work, it's easy enough to take off the new m/c off for bench bleeding).

Is the reservoir just a push fir into the m/c?

Cheers

John

 

[jaymanek]

Yes push fit... im always scared of the fittings shearing off the tank but hasnt happened yet!

Ive never pre bleeded... probably good just to put the pressure bleeder on, open the bleed screws on the m/c, save so much air travelling down the lines.