Yet another “spongy” brake problem. I have read the other posts here on the forum and they were helpful, but I still have a problem with my brakes. If anyone has any advice I would greatly appreciate it.

Here are the particulars of my truck. 1992 F-350, Crew Cab, 4x4, 460ci, 5-spd, w/230,000 miles.

Symptoms, with the engine running the brake pedal will slowly go to the floor. With the engine off the pedal feels normal (cannot be pushed to the floor).

During a normal brake job I replaced the following.

1. New calipers

2. New flex lines (from steel line to caliper)

3. New Wheel Cylinders

4. New shoes

Next I bled the brakes with a power bleeder. Thinking I was finished with the job, I started the truck and much too my surprise I have a “spongy” brake pedal. Bled another qt. of fluid out of the system, still “spongy”. (Note, I bled the brakes starting with the wheel cylinder furthest from the MC and moving my way to the closed bleeder valve).

Then my son helped me bled the brakes the old fashion way (pump the brake pedal, hold it, crack the valve, push pedal to floor and hold, close bleeder valve). After this I still had the same “spongy” pedal. The end result of all this bleeding activity was clear brake fluid (no air bubbles) at any bleeder valve.
Next is the list of all activities I did in hopes of fixing this problem.

1. Checked all fittings and could not find any leaks.

2. Adjusted the rear brakes so they are good and tight. This helped a little.

3. Bypassed the RABS valve. Note: Ford sells a bypass (Part Number F6TZ-2C280-AA), it is approx. $30.00. No change in pedal feel.

4. Checked the vacuum level coming into the brake booster and it was 21.0”Hg.

5. Pulled 10.0”Hg of vacuum to booster and it did not leak down.

6. Measured the height of the push rod coming out of the vacuum booster and it was at the low end of the tolerance, but still in spec at .980” (Tolerance is .980” - .995”).

7. Replaced the MC with a remanufactured unit. The new MC was bench bled. During the bench bleeding process I only stroked the piston assembly no more than .5”. I was told that stroking more than 1.0” would potentially cut the seals as they move past the machined ports.

After all of the above my I bled the brakes manually. Still the pedal feels “spongy”.

Could I have purchased a defective master cylinder? I find it hard to believe, but maybe I did.

Does any one know where I can purchase some plugs that will thread into the threaded ports of the MC? I want to be able to leak test the MC. The auto parts expert that I deal with only sells fittings, but no plugs. He says that no makes any plugs.
I apologize for the length of this post. Any thoughts, suggestions and/or ideas?

Thanks for your time,

John

 

Did you "bench bleed" the master cyl. before installing it? Try that, otherwise it wouldn't surprise me if you got a bad rebuilt. Years ago, I went through 3 before I got a good one on a 1986 Bronco.

 

RudyF6. Thank you for your reply. Yes I did bench bleed the MC. I did not over stroke the piston assy and before I installed the MC I could only stroke the MC 1/8".


John

 

You are not suppose to pump brakes to bleed them. You are suppose to open the bleeder screw and then have someone slowly push the pedal down, then close the bleeder screw and repeat. I cant tell you how many times people have had problems bleeding the brakes the wrong way. I rarely need a pressure bleeder when I bleed brakes.



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Daniel

2003 F-250 V10 Sport CC 4X4 Long Bed

 

Take a good look at you power brake booster I had a old power wagon that had this type of problem.

Good Luck

 

Sometimes if you leave your engine on while bleeding an older rig it will work.

 

First off there are differences from a spongy pedal and a pedal that is high but drops down to the floor with or without the engine running. Most times............ A spongy pedal usually is air and you can pump the pedal a few times to get it to come up unless you have too much air to compress then it will go to the floor. Spongy pedal can also be caused by mechanical issues such as bad calipers broken or bent pads and shoes and hose issue's. Also silicone brake fluid is spongy. If all the air is out of the system which is hard to confirm sometimes and you have a pedal that goes to the floor the problem is usually the MS but not always, many times its a mechanical issue that mimic's air in the system. The best way to test for this is to block and cap lines off to see if the problem goes away. You can get fittings and line , cut the line and crimp and braze the ends. You could solder also..As an example the rear shoes could be bending causing the issue.. Hope this helps

Dick

 

I had a 92 F350 same thing. It was the RAB's valve( Rear antilock Brake ). They say you can't rebuild, but I took mine apart cleaned out the gunk, and it worked. It's located under the truck about halfway back. It has a electrical plug on it as well as brake lines.
Just a thought
Tim

 

Thanks everyone for the advice! It is greatly appreciated.

RamboV10 - I did not know that pumping the brake pedal before cracking the bleeder valve was the incorrect procedure. I will try it your proposed method.

99f350sd - I will try blocking off the ports at the MC next.

Thanks again,
John

 

Rambo is correct, Crack the bleeder slightly, hold finger over bleed port, slowly press pedal to floor. Close valve, repeat procedure until clear fluid and not bubbles (you can fel them). If you have antilock, bleed this PITA first, then the right rear, left rear, right front and the left front. You may need to go around two or three times to get it right.

A little trick I learned is to collapse the calipers all the way down and wedge them tight so they don't move. This applies to two wheel disk or four wheel disk. Watch the master cylinder all the time. On the last bleed, remove one wedge at a time (start at the left rear), press the pedal and fill the caliper and release the pedal, then bleed. Also, if you are going to this point, do a complete flush. Use a BRAND NAME FLUID, not a Walmat, Autozone or NAPA special. They may be the same, but....... If you still have a sponge, I would suspect the Master Cylinder. You should not have ove 3/4 travel before the brakes start working. Also with rear drum brakes, tighten them up until you cannot turn the wheel by hand. You can always back them off after your are finished.

 

Okay, first thanks for all the help. I am closer but not quite there yet.


By plugging off front & rear and left & right wheels I have narrowed down to the right front wheel. (When the right front is plugged, the pedal is normal).

I have bled 2 qts through the right front to no avail. I even took the caliper off the rotor (placed a hard spacer in between the pads that is the same thickness as the new rotor) and bled it upside down. There is not a leak were the flex line attaches to the caliper. I also isolated the right front system, pressurized it, sprayed the entire system down with soapy water and saw no bubbles.

I almost convinced myself that the new flex line was bulging alittle bit, but the more that I played with it, I changed my mind. I think what I was feeling was the line attempting too straightening out when pressure was applied.

Anyone have any ideas? What am I missing?

Thanks,
John

 

I was thinking flex line also. Take it off and plug the line. Take no prisoners..

Dick

 

Shoot!! I was just reading everyone's advise again and I realized that I did not try pmasley's trick of collapsing the piston in the caliper, then bleeding the system. That is next. I just have too round up my 8 year old again to help with the bleeding.

All let you know how it turns out,
John

 

With new pads its collasped anyhow isn't it.??



Dick

 

Yes the calipers/pads are new but the pistons in the caliper still collapsed about 1/8". Bled the brakes. No bubbles in the fluid. Reinstalled caliper, no change.

Tomorrow I am going to replace the flex line. Does anyone manufacture steel braided brake lines for a 92 F-350? Maybe that is unnecessary (steel braided brake lines that is)

Thanks,
John