Porsche Enthusiasts Club Forum

I should have been clearer!

Corrosion in the master cylinder bore is inevitable, as the fluid attracts water which corrodes the unused section of the bore (you never normally push the twin cylinder all the way in).

When people bleed brakes using the two-man pedal method, they may allow the pedal to drop into that corroded section if the bleed valve is held open too long. As the seal passes the corroded section it gets damaged, after which it will only seal fully against the bore if a lot of pressure is applied to make it flare out enough. Light pressure will result in a poor seal, fluid flowing past the seal, and a pedal which is far too soft. It isn't that the corrosion prevents full movement of the pedal, it's that the bleeding process unless it's done properly will allow the pedal to move into the corroded section. I prefer to use a power bleeder, which puts the reservoir under pressure, with no need to touch the pedal.

Agree that after 10-15 years or so all master cylinders will show this corrosion. Normally it's no problem at all, but if you want to avoid any risk of the cylinder going bad during manual bleeding then it should really be replaced.

I not long ago refurbished my master cylinder, no rust or corrosion inside but quite a bit of wear,

Quick whizz with a cylinder hone new seals and it was fine,

As a rule if I can't verify regular brake fluid changes then they all get stripped and rebuilt, there is nothing worse than putting your foot on a brake pedal that suddenly ceases to function

It invariably ends with a clothing change

My Audi, Fiesta and BMW bike all have a brake fluid change every 2 years in their service schedule...

Regular - preferably annual - flush will keep MC in great condition. But with older cars the damage can have been done in earlier life before our ownership.

'Tis an interesting question; how often to change the brake fluid. But it's usually asked because the fluid degrades over time, not to prevent corrosion of the master cylinder (which is something I'd neve thought of).

I change the fluid in mine every two years. Maybe more frequently would be a good idea.


Oli.