Bleeding master cylinder in the car

Joey4speed said
Did you hear about moisture issues when using DOT 5? It's inability to absorb moisture causing corrosion and brake fade due to water puddling?
I use any DOT fluid except 5 for that reason.

The main issue with DOT5 is mixing it with DOT2 or 3. DOT 2 and 3 can absorb moisture. So while the different DOT fluids are compatible, what happens is the old fluid has moisture in it and the DOT 5 can’t, so it can create “bubbles” of trapped old fluid which has moisture in it. Since the fluid doesn’t move all that far it can rust out a localized section of line, if left alone long enough.

In theory, at least.

It’s why DOT5 is recommended for new systems and not as much for upgrading old systems unless you really clean the old system out.

sireland67 said
The system is not going to be completely full of fluid, because for brakes to work, fluid leaves the master cylinder, when the brake is pushed, filling the wheel cylinders, and calipers full of fluid, which stops the car.
When the brake petal is released, the fluid travels back to the master cylinder. The wheel cylinders, and calipers will not be full of fluid and the car moves.
A cold system, when heat is introduced, will sweat causing condensation.
Its simple physics.
Why will a good looking brake line, actually break, because it rots from the inside out.
DOT 3 or 5 will work, if you have replaced every component, at our age you will not have to worry about it.

Real qualifications, I am a degreed engineer, and have worked in industry for over 30 years designing aircraft.

What?

Good grief man. The calipers and wheel cylinders are always full of fluid in a properly bled brake system. The fluid displaced by pushing the pedal is moved from the MC through the lines to move the pistons in the calipers/wheel cylinders and returns when the pedal is released. The only part of the system with an air gap is the master cylinder above the level of the fluid.

The fluid is in incompressible, otherwise none of it would work. Air is compressible, which is why you have to bleed all the air out for the brakes to work. No void space is ever created in the system below the MC, the pistons move back and forth proportionally to the fluid displaced and everything below the level of fluid in the MC remains full of fluid.