Brake Service – Protect master cylinder

[ReV]

Hi, I’ve watched Eric’s video demonstrated how to replace a brake rotor.
In his video, he use a plier to hold the hose, he say that will prevent the fluid running back to the master cylinder.
May I ask why? If the fluid run back to the master cylinder, will it do any harm? (I think they are the same fluid anyway)
Thanks!!!!

Topic

[Joe]

I was hoping to find this out myself, Hopefully someone will slap us an answer soon. banana:

[Jared Thomson]

I am assuming this is because the brake fluid inside the caliper, especially after it is disconnected, may contain rust and other contaminates you probably dont want in your master cyl.

[Joe]

Thats a pretty good point, Looks like I’m gonna have to grab a tube and a bottle of mountain dew. 😛

[WDHewson]

XX

[EricTheCarGuy]

The reason is because of the cup seals in the master cylinder. They are designed to move and seal in one direction only. If you force fluid past them in the opposite direction of how they are suppose to work you can unseat them thus ruining the master cylinder. I’ve seen it happen on more than one occasion which is why I show the methods I do when performing a brake pad replacement. The old fluid has NOTHING to do with it.

A secondary reason is that often when you do this you overflow the master cylinder thus running brake fluid all down the firewall. Not only do I consider this unprofessional, but it also ruins paint.

[Author]

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