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Q: Brake pedal spongy and slowly goes to floor on 1998 Ford E-350

The brake pedal is spongy and under pressure goes to the floor. I also notice when I have a helper depress the brake pedal while I look into the brake fluid Reservoir, the fluid boils up. No air, just a backflow of fluid?

This is the Vacuum booster model. Dually, rear ABS only.

Replaced pads, rotors, rear shoes.
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Looks like a master cylinder problem. It won't hold pressure and the fluid flows back to the reservoir. If you replace the master cylinder, get the whole brake system flush/bleed out with new brake fluid. You should do this every 2 years or more often if you live in a humid area. Better to bench bleed the cylinder before installation.


Thank you ZeeTech.

I've bench bleed and replaced the MS. I still see the slight boiling of brake fluid in the reservoir. I've run a QT of brake fluid thru the system and the brakes are still very spongy?
Check the reservoir's cap, make sure it breathes ( the little vent hole isn't clogged).
Some Fords are hard to bleed. Did you bleed only at the MC or at the wheels also?
When you replaced the rear shoes, did you lift up the rubber dust cap from the wheel cylinder and checked for leak? Absolutely no brake fluid leak from the system?

Hello Zee,

I've bleed all the wheels twice and replaced both the rear wheel cylinders.
No loss of fluid I can detect.
I suspect your right about difficulty bleeding the system. I'm taking the truck to a pro in the morning to bleed the system using the vacuum method.
I'll report the results.
I have a simalar problem. I have replaced the rear calipers in my 2005 f450 motor home and cannot get the brake pedal from goining ot the floor. Worst when it is running. It slowly goes to the floor and slowly comes back up. I have bled all four tires (about 2 qts of fluid). Can't believe there is still air in the system. I did run the fluid pretty low on one occasion and could of sucked more air in. Should I be looking somewhere else or just keep bleeding?
I ordered a power bleeder, and am waiting for it to arrive. The Ford dealer wanted $135. Will report back. The only item left to replace is the ABS modulator. Your 2005 might need a "Scan Tool" in order to bleed your ABS system. My truck is older and can be bleed wirh out this tool.
same problem here too 94 E350 I have replace front pads rear shoes checked all cylinders replaced master cyl bypassed ABS still pedal goes to floor it just started doing this out of the blue I have 7.3 IDI engine if it is not running pedal stays hard and wont fall at all
I had the same problem with my E350 rear brakes after changing my brake shoes. I found two things that helped me with the problem. There is a rubber winding hose that travels down to a block on the axle. I straightened that hose as much as possible to free up any air pockets that may have been trapped in it. Secondly, I bled the brake line fitting at the highest point above the axle, where the rubber hose and the steel tube meets. After this, I bled my wheel cylinders and that worked for me.
the abs module is located drivers side on the inside of the frame just back from the front wheel.
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