Why does the brake pedal on a 74 f-100 go to the floor when the truck is running?

Why does the brake pedal on a 74 f-100 go to the floor when the truck is running?
Usually when the break petal goes all the way to the floor, it indicates a leak in the break system. The most common causes are:

no break fluid in the master cylinder due to a break line opening or a leaking valve in the master cylinder itself.


if it is a power break system, then the vacuum assist may be leaking. There is a small (usually white) valve with a vacuum hose on it (about half inch) that tends to go bad and causes the petal to go down to the floor. Its typically an easy fix, an inexpensive.




Good Luck.