On my '92 F-350 (in sig), I have a really soft and spongy pedal. I have replaced the front pads, master cylinder and have bled and bled and bled the brakes. Rear shoes and all hardware are new. They are adjusted properly. When the truck is off I have a rock hard pedal and it doesn't sink. Fire up the truck and the pedal acts normal but becomes spongy and with foot on the brake it slowly creeps to the floor. If you press hard enough it gets to the floor pretty quick. The parking brake works good and only goes about 1/3 to halfway to hold the truck on inclines.

 

Did you bench bleed the master cylinder? Because it's nose-up when mounted, air gets trapped and you'll never get brakes no matter how much fluid you run through it.

Did you bleed the ABS valve?

Tons of information in this forum on both of these topics.

 

I bench bled the master and did bleed the RABS valve going RR, LR, RABS, RF & LF

 

Brake pedal is rock hard when truck is off and as soon as Booster is applied with vacuum I get the soft/ spongy pedal. Sometimes its worse then others and usually pumping will harden it up.

 

try bypassing the ABS unit. i bet you will find a rock hard pedal with it bypassed.
this will tell you the abs unit is bad.

 

About 2 weeks ago or so while driving on the freeway I had to slam on the brakes as traffic trying to bypass an accident was getting over. Immediately after that and for a week or so the pedal was harder and felt normal. Now it's getting back to its spongy self

 

If there isn't an external link but the pedal will slowly drop to the floor then the master cylinder is defective. Period. There's no black magic involved in these systems.

Do this: with the engine running, apply moderate pressure to the brake pedal. When (if) the pedal finally hits the floor, pump the pedal again and continue to hold the same moderate pressure. If the pedal repeats the decent, you have proof positive that there is a leak in the system. If you aren't losing any fluid from the reservoir, then it's an internal leak and it's in the master cylinder.

Defective "new" master cylinders aren't (unfortunately) rare. You aren't the first to be subjected to this, and certainly won't be the last. Be sure to give the vendor the appropriate amount of crap.

 

Usually after limping the pedal gets firmer. I replaced original master cylinder with new parts store brand and same thing.

 

How does the term "limping" apply to brakes?

If your pedal is hitting the floor as described and you've done my proposed test then the master cylinder is bad. Where else would you suspect the fluid is going?

 

Couldn't it be filling the accumulator in the RABS module?

 

That accumulator ain't that big... it would fill up pretty fast. Probably before the pedal even got to the floor.

Do the test cajohnson suggested. Just cause a part is new doesn't mean it's GOOD. Parts store might have sold you a junk MC in which case the test will find out for sure and if so you can just go back and return it for another.

EDIT: tjc also has a good point though. I don't know how the internal valving is set up in the RABS valve body, but if it failed in a way that allowed it to bleed off pressure then that could cause the symptom you have too. It wouldn't be filling the accumulator, just rerouting fluid back to the low pressure side.

 

Ok I will try it. Pedal never reaches the floor but it gets close if you're sitting at a light with a good amount of pressure on it

 

I reckon he meant to say PUMPING. Makes more sense, anyway...

 

Indeed.

Seems only reasonable that folks looking for troubleshooting help on their vehicle might make sure we don't have to troubleshoot their statements first.

 

Sorry autocorrect. Good thing Dixie understood then.