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JoeW
07-27-2017, 12:11 PM
I am getting ready to fill the brake system on my 57 Golden Hawk and notice that the fill cap on the master cylinder is vented. Since the hydrovac unit is higher than the master cylinder, what keeps the brake fluid from leaking out of the master cylinder cap by force of gravity?

Dwight FitzSimons
07-27-2017, 12:29 PM
There is no reservoir of brake fluid higher than the master cylinder. When you push on the brake pedal it pushes fluid into the brake lines. The Hydrovac is in-line with the brake fluid lines and adds pressure to the pressure applied by the driver. That is, the Hydrovac pushes the fluid in the direction of the wheel cylinders, not back toward the MC.
-Dwight

bezhawk
07-27-2017, 09:08 PM
Drum brake master cylinders have a residual pressure check valve in the cylinder. A couple of pounds is required to overcome the valve. Gravity won't do it.