It's probably different for every truck. But, if you have a wheel cylinder leaking, fix that first. On mine, I would use it maybe once a week if not less. Every time I would go get it, the master was dry and the pedal went to the floor. An external leak that big would show itself in a big way. I didn't have time to replace it at the time so I by-passed it and I haven't lost a drop since.
Scott
