1994, F350 Crew, 4x4/><O></O>
Symptoms... <O></O>
Spongy brake pedal after applying brakes with engine running... vehicle stops but the holding pressure causes pedal ultimately to go to floor. <O></O>
Narrowed down spongy pedal ... between where front brake line connects to Master Cylinder to front Calipers. <O></O>
<O> </O>
Process of elimination<O></O>
1. Installed new rubber front brake lines... one showed dimples. <O></O>
2. Installed new Master Cylinder... completely flushed system... bled all lines in sequence order along with abs control valve.<O></O>
3. Removed both brake lines and plugged both ports on MC ... pedal held firm... with and without engine.... which suggests MC is functioning without issue nor internal leaks. <O></O>
4. Plugged off front port line on MC.... installed rear line. pedal firm and held<O></O>
5. Plugged off rear port line on MC... installed front line. Spongy pedal symptom occurs. <O></O>
Bled lines during each above process to assure clearing any possible air issues... <O></O>
<O> </O>
Before removing/replacing calipers.....

• Rotors show no signs of issue, Pads at about 80%, outwardly everything appears in good shape.
• Vehicle initially stops and holds without any pulling, grabbing or other signs of failure. <O></O>
• When the pedal does end up slowly meeting the floor... holding it while giving additional acceleration .. provides significant torque effect.. but no travel forward.

<O</OThis suggests to me there may be some sort of internal leak in one of the front caliper cylinders. ?<O></O>

 

No on an internal leak. Are you sure the front line goes to the front brakes? I've seen RABS valves leak internally and cause a sinking pedal.

 

yes ... followed line from forward most port on MC down to a T ...where one end of the T connects to left front rubber brake line leading to the caliper... while the other end of the T leads over to the right front.

 

It sounds like an issue with your master cylinder. When the pedal goes to the floor can you let up on it, pump it again and have a decent pedal until it goes to the floor again?

Did you bench bleed the master cylinder before installing it?

 

To ansewr your question about the caliper having an internal leak the answer is no. The inside of the caliper is just a big void with either 3 or 4 hole in it. One for the bajo bolt holding the hose on. one for the bleeder screw, and either 1 or 2 for the caliper piston depending on if you have single or duel piston calipers. Are you losing fluid level in the Master cylinder? Not sure where you are losing pressure because you already replace the brake hoses.

matt

 

I think you pretty well eliminated everything but the calipers. There's no place inside a caliper for fluid to leak to but past the pistons and into the dust boots. Eventually those would fill up & would leak to the outside.
These calipers are notoriously difficult to bleed, though. Every time I've opened the front brake system on my F250 or F350 I've ended up with a mushy pedal after bleeding tons of fluid. Now I bleed them, go for a drive on a washboard road, and then bleed them again. Every time I end up getting more air out, and eventually get a good pedal.
There must be some cavity in the caliper where air gets trapped...

 

Yes... however .. as referred in the posted process...
When plugging Both ports of the MC and applying the peddle.. all is as it should be.
The peddle is firm... with no spongy effect ...nor does it head to the floor.
From my limited understanding... if there is an internal seal leak in the MC....the fluids leak between themselves...
Thus upon plugging both line ports ...and applying the brake peddle... if the peddle presses down and then stops at what would be a normal stopping position... remaining firm and not progressing any further... would that not indicate the MC is working properly and the seals are good?

 

no signs what so ever of any fluid loss.. in the reservoir nor anywhere else.
thanks for the input everyone... helps my noggin to focus better
Eliminating from my chk list the plausibility that Calipers cannot invisibly leak internally helps.
Will try the re-bleeding washboard process to see if that may cure the dilemma.

 

The way i bleed/flush my brakes is I have a 10' piece of clear tubing, after I hook it up to the bleader screw and open the bleeder I have a helper pump the breaks while i monitor/maintain fluid level. I run that tubing up above the level of the master cylinder and into a jug(2 liter bottle). That way i can watch for air or fluid discoloration and make sure I get all of the old fluid out. This is a very mind boggeling situation.

Matt

 

Yes the MC was successfully bench bled...
Moreover.. to further test (rule out the MC) just swapped it out with our other F series... aka uses same MC... results.... using that MC in the other vehicle ...all is working as it should.

Appreciate the feedback.

 

Also was the brake fluid you used from a new unopened container?

A previously opened container can contain moisture that will cause similar problems even if the fluid still looks good

Matt

 

If the pedal holds then your MC is working properly. If it continues to travel to the floor then most likely there is air in it. Not sure how you're bleeding the brakes but I do mine at l/r, r/r, r/f, l/f, pump the pedal three times, hold it and open the bleeder. I'll do that several times before moving to the next wheel. It's a pain, I miss having a vacuum or pressure bleeder.

 

Matt... I too use a clear tubing into a plastic water bottle to bleed..
Great way to both monitor the flow and provide a visible control of any variances. Moreover the ability to contain and isolate evidence from each bleed.
Thanks

 

Matt.... Using New gallon of Dot3

Husky.... the only additional bleed from your input is after I do the l/r, r/r I then bleed the abs controller valve before going to the front.
Though I havent done the 3 pumps first process... will try that ...

 

Finished driving it around ... for the suggested rumbling effect
Also tapped on caliper cylinder area while once again bleeding.<o></o>
Helper pumped pedal 3x upon 'up' command.. before each opening of bleeder. <o></o>
No improvement...<o></o>
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I will add.... three other circumstances noted during all this..<o></o>
<o> </o>
To clear up any potential confusion as to the pedal performance.<o></o>
Upon applying..whereby when it becomes firm at stopping the vehicle... once pushing on the then spongy peddle... the pedal actually never reaches the point of touching the floorboard. It stops and holds firm about 1-2" from it. I mention this because ... of noting among other postings this hasnt been typically made clear.<o></o>
<o></o>
The other notability is that when bleeding the l/f I noticed... unlike any others I have bled now or in the past... that when even opening the bleeder valve a small amount... fluid noticeably begins leaking from/through/around the treads themselves. <o></o>
<o> </o>
Placing the tubing over the end of the bleeder to view the flow and possible any signs of bubbles...continues to result in a clear flow. aka no bubbles. <o></o>
Even though there is this leaking at the threads... I cannot surmise that any air would be entering the system from that point... being the valve is closed before the pedal is released.<o></o>
<o> </o>
Lastly with regards to any possibility of caliper internal leaking issues... <o></o>
The calipers noticeably have dual cylinders ... with that said.. would/could there be any sort of seeping going on between the two. Thereby causing the air-pocket symptoms??? <o></o>
Merely a FYI .. for anyone else who might be similarly situated <v:shape style="WIDTH: 11.4pt; HEIGHT: 16.8pt; VISIBILITY: visible; mso-wrap-style: square" id=Picture_x0020_72 type="#_x0000_t75" alt="0" o:spid="_x0000_i1025"><v:imagedata src="file:///C:\Users\AJP\AppData\Local\Temp\msohtmlclip1\01\cl ip_image002.gif" o:title="0"></v:imagedata></v:shape><o></o>