I am working on a 1995 F-350 4WD Dually Diesel. I replaced all of the brake hoses and steel lines, new calipers, pads, rear drums, shoes and wheel cylinders and a new master cylinder. Bled the brakes and got all the air out. When you start the motor the brake pedal goes right to the floor. Motor not running works as it should, motor running pedal goes right to the floor. I have already changed the master for another new one. Any ideas as to what I am missing? Appreciate your help.
Bleed them again and make sure you got brake fluid going into the lines . When you pump them up do they get hard and then go to the floor or do they not get hard at all. If they do not get hard you're getting air into the line somewhere and I would really examine the master cylinder and the lines going in. It may not have gotten a good enough prime. The same thing happen to my 81 F-100
Don't knnow if this will help, many years ago a friend of mine did a brake job on his f-100 and had the same problem. I took the front wheels off and discovered the calipers were on upside down, the bleeder was on the bottom instead of the top. He had switched the two sides. He could bleed and get fluid but since the bleeder was not on top he couldn't get all of the air out.
You still have air in the lines. Have a look on the inside of the left side frame rail for the RABS module (if fitted) and bleed that after the rear brakes. It`s about level with the brake pedal.
the RABS is right below the master cyl on the drivers side frame. also if you take your master cyl off is there fluid leaking past the diaphram/plunger
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90 f250 4x4 5-speed 351w headman LT's 33x12.5 coopers CHEVROLET-Cheap Hardly Efficient Virually Runs On Luck Every Time
Yes the master cylinder was bench bled and the RABS valve is bypassed. By that I mean the rear brake line runs from the rear axle to the master cylinder. This is an old township truck...think salt and corrosion!! That is why everything needed replaced. Thank for the replies!!! This has me completely befuddled.
OK, now don't get upset here folks, I'm a Toyota tech for a new car dealership. On some of the older cars, we experience a similar situation when the caliper sliders stick. Pump all day long with good fluid flow and a hard pedal, but start the engine and pedal is on the floor. Took a long time to figure that one out.
You need a different MC if you bypass the rabs. RABS works by bypassing fluid pressure back to the resevoir. 1 line from the res in an input and the other the output. You are basically dumping your high pressure fluid right back into the res.
Well at least thats my understanding of how our RABS works. Try using an MC from something like a 70's truck.
Just get a MC from and F450. I had to do that on my 91 Diesel F250. There was actually a service bulitin about it, that was the repair. Now the two mounting holes where it mounts to the brake booster, you have to elongate the mounting holes in the MC to fit, but all else is a direct bolt.
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1997 Ford F250 4x4 
