I have a 1990 F-350 460 4X4 Crew Cab. The brake pedal goes too close to the floor. I have adjusted the rear brakes, installed a new master cylinder (went bad a year ago), bled lines in correct order, 1/2 pad life in front. The booster does hiss when I shut the truck off. I believe it is the original booster from the factory. Could this cause a low pedal? What is the RABS Valve? Does that have to be bled? Any input would be great.

 

my buddy has the same thing going on in 1992 f250 put new shoes on front and bleed the brakes all around and still nothing.I was thing to put in master but you put one in and still the same thing that sucks hope you can find the problem let me know what you find out and ill let you know what i find out thanks

 

Your booster may or may not be bad but either way, it will not cause a low pedal.
The RABS hydraulic control module is supposed to control line pressure to the rear brakes to prevent lockup under heavy braking. You need to bleed it just like you bleed a brake (bleed screw on it). It is bolted to the inside of the left side frame rail about level with the drivers feet.

 

I take it that bleeding the RABS valve is the same as a wheel cylinder? Thanks for the responses.

 

Also, are there any tips I should know about when doing the booster? Thanks

 

There is a bleed screw on the RABS module and you bleed it just like a wheel cylinder. The correct bleeding order is right rear, left rear, RABS module, right front, left front.
Booster should be straight bolt in. You should be able to move the master cyl forward enough while leaving the pipes connected.