Two problems related to power assist brake have been reported for the 1999 Ford F-350. The most recently reported issues are listed below.
Hydro-boost assembley started leaking at master cylinder and hydro-boost mount point. Lost of brake assist and steering due to fluid level loss. Ford dealer has truck for repairs repalcing hydro-boost assembly. I see this is the third reported incident on the web page.
Brake system failed. Also, steering system failed due to a faulty hydroboost unit that controls brakes and steering.