No brakes on 03 7.3

[P&DDennis]

I have a 03 F550 with the 7.3. I am having an issue with the brakes. Pedal was soft and spongie. Checked the master cylinder, It was full. Was told it was the master cylinder. Replaced it today same problem. With the truck OFF you can get a good pedal, Once started it soft and spongie. Has anyone every had this problem?

[snowjoker]

I bet its your Brake Booster ;)

[wmhlc]

I'm no master mechanic or anything, but I'm pretty sure the 7.3 uses the power steering pump to build pressure. I would check and make sure your power steering fluid is filled and not low, and you have no leak in the lines.

I had the same problem on a truck once. We hit the power steering fluid line when attaching the plow and put a small hole in the line. That was enough to kill the brakes.

[P&DDennis]

It does use the power steering pump and it is full.

[P&DDennis]

Has anyone ever had to replace a booster on a newer truck?

[john mc]

The booster sounds like it is doing its job .You said when you have the truck off the pedal gets hard . Then when you start it the pedal gets soft . The booster is giving you more hydraulic pressure that is why it is getting softer.you something else is wrong. If the booster was bad it would feel like it was off when you start the truck. I hope that makes sense . you need to pull the wheels or look for leaks

[chtucker]

Sure its not one of your rear pads that have broken up? The rears have a bad habit of losing pads... it will give you the feeling of no pedal

[EIB]

seized pistons in the calipers, and seized calipers slides will cause the problem you describe.

[P&DDennis]

I have no leaks. But will pull the calipers off on Monday and check the slides and pistons

[Pickering Snow]

You didnt get the air out of the system go back in and bleed the master on the truck and then the furthest rear wheel working forward to the master til all 4 wheels are bled.

The hydro boost is not the problem if it was you would have a rock hard pedal meaning no power assist.

Again proir to the master replacement and iam sure now air in the lines you should inspect pads , caliper, and hardware since yes it could have been a master but i suspect a underlying problem.

Most dont know how to bench bleed a master right hence once installed on the truck you add even more air to the lines and system so do the first thing i said first.

[P&DDennis]

I am betting on a frozen caliper or piston. Did the bench bleed with the hoses in the master and pushing in on the master until both lines had no air, What a pain to change. Realized after the fact that the battery and air box needed to be removed to get the master out.
Sometimes I find myself to close to the forest to see the trees. We have a big storm brewing for tuesday night and brakes will help out a little?

Thanks everyone for their input and I will update on what was found. Cant wait to remove those rear duals. Did never seize last winter, Hope it worked.

[L-ENTERPRISES]

Does'nt the dealer need to use a computer to reset the abs before you get brake pedal back after bleeding the system ?

[wyldman]

No need to pull the wheels to bleed, you can get at the bleeder screws with the duals on, it's just a PITA.

[chtucker]

Curious minds want to know... What did you find?