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Chris (aka- Moose)
Chris (aka- Moose), Technician
Category: Ford
Satisfied Customers: 44460
Experience:  16 years experience
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98 E350 7.3 brakes fade when running. When not they seem to

Customer Question

Customer: ***** *****, 98 E350 7.3 brakes fade when running. When not they seem to be fine. Can the vacuum booster cause brake fade?
JA: Thanks. Can you give me any more details about your issue?
Customer: I have replace a faulty wheel cylinder and just replacing the master cylinder.
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Submitted: 11 months ago.
Category: Ford
Customer: replied 11 months ago.
When I say fade, I mean, while sitting, with engine running, foot on the brake, the pedal slowly goes to the floor. There is not leak in the brake system that I can see. No wet spots at the wheels or on the frame. Not dripping from the frame or wheels. When the engine is not running the brakes seem to stay firm.
Customer: replied 11 months ago.
One more thing to add. While bleeding the brakes, toward the end of the stroke there is a whistling sound. (A leak from the booster?)
Expert:  Jeremy replied 11 months ago.

My name is ***** ***** I will assist you.

You are completely describing a failed or leaking brake booster.

I would change the brake booster out. The telling sign for me was your description in how the pedal slowly goes to the floor with engine running, but doesn't when it is not running. If it was a fluid leak, it would go to the floor at all times. You only have it going to floor when running so the pressure in the booster is leaking off.

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Jeremy

Customer: replied 11 months ago.
Could there be air in the proportioning valve? And with out running there just isn't enough pressure to make it fade?
Expert:  Jeremy replied 11 months ago.

There can be, yes, but does your brakes feel like you are not stopping like you should? Usually when there is air there, you feel like you are not stopping and not just the pedal going down. You go down a little bit and stop when there is air. You don;t keep going down and down and down.

Customer: replied 11 months ago.
What is the "valve" that is mounted under the master cylinder? Are there 2 proportioning valves?
With air it just is spongy but shouldn't go to the floor.
How does the vacuum booster work? Is there a direct link to the master cylinder?
Customer: replied 11 months ago.
I have resisted driving it much to see how it brakes while the pedal goes to the floor. Maybe I should try a short slow trip to see how it responds.
Customer: replied 11 months ago.
Is there a way to concretely prove that the booster is leaking?
Expert:  Jeremy replied 11 months ago.

Spray soapy water on the booster and have someone press on the brakes. You will see bubbles.

Do you have rear wheel ABS on the van? That valve is for the ABS if you do.

Customer: replied 11 months ago.
Trying to determine if there is ABS, there is an ABS light on the dash. How do you bleed the ABS valve?
Customer: replied 11 months ago.
I was told that air in the abs valve would cause the pedal to drop and that it must be bled.
Expert:  Jeremy replied 11 months ago.

Correct. You will need a scanner for that to activate the ABS pump. Autozone will have one for free. You just activate the pump, crack a bleeder valve, and it will push the air out on its own, then you bleed like normal from there.

Expert:  Jeremy replied 11 months ago.

I see you viewed this and this is still showing as a open question due to no positive (3 stars or more) rating.

Did you still need assistance with this?

Customer: replied 11 months ago.
Still have the problem. AZ doesn't have the tool and I don't know where to get one. It seems I need an alternative way to bleed the system and I can't find it. It would seem that soapy water in the vacuum booster couldn't create bubbles because it is a vacuum not creating pressure. It seems like guessing not knowing how to solve the problem.
Expert:  Jeremy replied 11 months ago.

I'm opting out. Booster will create bubbles if leaking for when you push in and let of the brakes, the pressure goes back out of the booster.

On the tool, every single Ford shop has a IDS and nearly every scanner with ABS can activate the pump for a bleed.

Based on your response, you are not skilled in doing the work or you are trying to get a free answer. Both are fine. I just will not help you for your response and comment is false.

Good day.

Expert:  Chris (aka- Moose) replied 11 months ago.

Welcome, I'm Chris (aka Moose) I'm an independent contractor here at Just Answer.

Different tech here, would you like me to try and help?

Thanks for asking your question and letting me try to help you.

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