I have a 1988 f350 dually with 7.3 idi. When I push on brake pedal it goes straight to the floor with no resistance won't stop truck either. The whole system was bleed a few years by my step father but still having the problem. Can any help me solve this problem before I go on total over haul of brake system.

 

If you have no leaks in the system at the wheels pistons and pucks and also are not loosing any fluid in the reservoir I would go with a bad piston seal in the master cylinder. You likely could get an overhaul kit and hone the cylinder before rebuilding it or just get a new master cylinder.

 

I will just replace the master cylinder and see where that gets me. What do you think about the rabs valve?

 

The RABS valve should be good. I have not heard of a case where they go bad but it could happen. However if you are referring to bleeding the system I have always been partial to pressure bleeding from the master cylinder.
Make sure you pre fill and bleed the master cylinder on the bench before installing it.
The pressurized method lets the job be a one person task and insures the system is flushed completely if you use a good supply of fluid.
As with normal two person bleeding start from the furthest point and work your way to the top. I would recommend a few times through to insure clean new and bubble free fluid in the system.

 

Okay thank you very much this problem has me stumped so I am very grateful for the help.

 

The RABS can actually be an issue if the dump valve doesn't close properly. The yellow warning light on the dash should be on cause of this, but it may not always be the case. IIRC the piston in the chamber fed by the dump valve is spring loaded, so you can take the end cap off the RABS unit and put some plug or whatever in place of the spring then reinstall the cap and that's how you verify a leaky dump valve - the idea being that if the piston can't move the pressure can't drop so if this makes the brakes behave then you need a new RABS unit.