I have a '90 F350 7.3 crewcab dually. When you step on the brakes the you get a full pedal then with continued pressure it will bleed slowly down almost to the floor until it gets hard. The master cyl has been changed twice and we know it is not leaking. I have been told there is a dump valve leaking. Does anyone know what a dump valve is and where is it located? Has anyone had this same problem? I've been told it's common on this era Ford truck.
Jim

 

Not shure about the dump valve, but the brake system can be tricky to bleed. Check out this prev. thread, might help.
https://www.ford-trucks.com/dcforum/DCForumID39/3463.html
1989 Centurion Classic Bronco

[link:groups.yahoo.com/group/centurionconversions/|https://www.ford-trucks.com/user_gallery/displaythumbnail.php?&photoid=3476&.jpg]<-< Centurion Club
1979 Bronco XLT
https://www.ford-trucks.com/user_gallery/displaythumbnail.php?&photoid=3923&.jpg

 

The dump valve being reffered to is the antilock brake valve.On the 1/2 ton it is on the drivers side frame rail in line between the master cylinder and the rear brakes.It bleeds off brake pressure to the back wheels in the event of lockup.I guess it could be leaking down brake pressure to the back wheels.

Billy