Master Cylinder

[Logan541]

Ok just something that has been bugging me, My Dad who was an awesome mechanic in his day always told me (and a few others have as well) to never let the master cylinder go dry when doing a brake job, but nobody has ever told me why. I mean if it does go dry and air gets in the system can't you just bleed the brakes to get rid of the air in them? I am also not sure why you would have to bench bleed it if that happens I mean just have someone pump the brake and just keep bleeding, what the devil am I missing (Dad's answer has always been "Because, I said so" 93 years old, gotta love him. LOL) Also does any one know if D.O.T. 3 and D.O.T 4 can play together? Most say yes but a few have said no...Thanks guys

[matt5112]

On an older car without ABS its not a problem.

Bleed the brakes, and it'll be fine.

If its a newer abs system, you can run into issues.

Namely the only tool which can properly cycle the abs module is a tech II scan tool.

I've read DOT 3 and 4 use slightly different chemicals to perform the same job, some of which aren't compatible with older systems.

Its not that the fluids aren't compatible, so much as the newer fluids are incompatible with the rubber seals.

On that note, I ran DOT 4 for the three years I had the car. Never had an issue. However that does not mean it's safe or going to last.

It's more important you have fresh fluid in there to keep the boiling temperature high than it is to use a better fluid and never change it.