Soft brake pedal - Ford Powerstroke Diesel Forum
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post #1 of 9 Old 03-25-2018, 05:44 PM Thread Starter
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Soft brake pedal

1999 7.3 F-250 superduty with 360K on it. Been having issues with my brake pedal being spongy. Replaced the hydra-boost and made sure there was no air in the system. All brake components seem to be in good condition and no air in system.

While in park with motor running the brake pedal get hard. As soon as I put it in gear the pedal becomes spongy. Also while driving if I slam on the brakes they are responsive and pedal is hard but if I gradually apply the brakes the pedal is soft and goes almost to the floorboard.

Not a mechanic so understanding of entire system is limited. Could the RABS valve be the issue? Is the wheel speed sensor somehow tied to the ABS system? Power steering pump issues? With he symptoms it does not seem logical for the master cylinder to be bypassing or any of the calipers out at the wheel.

Ideas? Experiences with similar problems?
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post #2 of 9 Old 03-26-2018, 06:52 PM Thread Starter
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Pinched off soft lines at wheel. The master cylinder isn't bypassing. Am thinking its in my calipers now.

Also talked with a mechanic friend and he mentioned that there was a master cylinder update with a larger bore than stock was. Might this help out on a soft pedal?
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post #3 of 9 Old 03-26-2018, 07:08 PM
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Has the brake fluid ever been flushed? Look in the master cyl, if it's dark instead of clear it needs a thorough flush. Many people neglect to do it, when I bought my 6.0 I flushed it and it was almost black.

What brake fluid shouldn't look like...
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post #4 of 9 Old 03-27-2018, 05:02 PM Thread Starter
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Thanks. I will try a system flush and see how that works.
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post #5 of 9 Old 03-28-2018, 07:01 AM
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I had similar braking to what you are describing when I bought the truck and so far I've found a few issues which helped tremendously.

-Rear brake line was leaking at a join- I tightened connection and bled caliper
-Brake fluid was low, albeit clear
-PS Fluid was low.

The power steering in these trucks takes ATF and controls the brakes... I'll flush mine one of these days because the fluid does not look good at all, but filling the reservoir to the top helped the spongy feeling greatly.

I still need to pull wheels and do some more inspections and I'll probably bleed/flush the entire system before I decide I'm finished. I did already learn that when the craigslist ad says "new calipers" it really just means one new caliper.

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post #6 of 9 Old 03-28-2018, 07:25 AM
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Here is a thread with the same problem.
https://www.powerstroke.org/forum/99-...ft-brakes.html

I had same problem on my wifes F150 and it turned out to be the ABS HCU and ECU

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post #7 of 9 Old 03-28-2018, 07:53 AM
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Quote:
Originally Posted by shadmcd View Post
Thanks. I will try a system flush and see how that works.
Some guys make them but I bought a power flush tool at Harbor Freight for about $30. It uses shop air to crate a vacuum to pull the fluid out at the bleeders. Allows you to do it by yourself.
When I did my 6.0 I did it the old fashioned way, with my son pumping the pedal while I worked the bleeders. (that's why I bought this thing)

I used one of their coupons, but this is what I bought.
https://www.harborfreight.com/brake-...der-92924.html

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2005 Explorer XLT 4x4 Mountaineer,
Toyo ATs, Wife's grocery getter

1993 F150 4x4, 302 5 speed, winter truck


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post #8 of 9 Old 04-03-2018, 02:04 AM
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Quote:
Originally Posted by shadmcd View Post
1999 7.3 F-250 superduty with 360K on it. Been having issues with my brake pedal being spongy. Replaced the hydra-boost and made sure there was no air in the system. All brake components seem to be in good condition and no air in system.



While in park with motor running the brake pedal get hard. As soon as I put it in gear the pedal becomes spongy. Also while driving if I slam on the brakes they are responsive and pedal is hard but if I gradually apply the brakes the pedal is soft and goes almost to the floorboard.



Not a mechanic so understanding of entire system is limited. Could the RABS valve be the issue? Is the wheel speed sensor somehow tied to the ABS system? Power steering pump issues? With he symptoms it does not seem logical for the master cylinder to be bypassing or any of the calipers out at the wheel.



Ideas? Experiences with similar problems?


Soft lines can break down internally and contaminate the system. I had a dragging brake on my 2000 f350 and saw that the previous owner had replaced the whole left rear brake but not the line. My right side was dragging so I changed the 2 lines and did a good bleed and never smelled hot brakes again.


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post #9 of 9 Old 04-06-2018, 09:22 PM Thread Starter
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After some testing and diagnostics I was able to determine it was a faulty master cylinder. The soft pedal was there when pressure to the brake was applied slowly and steady. But when I hit the brakes a bit faster and harder I had a solid brake pedal. After changing out the master cylinder and doing a proper bleed of the entire brake system I once again had solid brake pedal and good brake performance.
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