Hello everyone,

So after many quarts of fluid and bleeding methods, the issue remains. The pedal feels normal for the first ~1/3rd of the push, then resistance fades and the pedal starts to sink.
I've found if as soon as I feel the resistance drop that I release and start over that it will actually "build pedal"
and feel as I'd expect them to, until I let go and the next stop sign is back to square one.

With the vehicle not running, I can apply steady force and the pedal will sink at a steady rate.
But what I thought was the booster making a "woosh" sound,
is apparently either air or fluid shooting up against the top side of the rear chamber of the brake fluid reservoir.

This can be repeated over and over until the pedal builds so high that I didn't feel comfortable pressing. Also, once the pedal is built up with the car off, I can stand on it and it will hold.
But let go for ~5 seconds and its back to slow steady sinking.

Did I fail to properly bench bleed the master cylinder? What could/would cause a burst of air to shoot out of the rear (covered side closest to the booster) of the reservoir?

Thank you!





I've spent the past 2 hours reading through 27 pages of threads on this forum for the search "brake pedal" and have not come up with a clear cut idea of what the solve is for my problem.

For starters the Van is a 2003 E350 XLT with the 5.4L, and I have owned it for all of a day.
So past maintenance history is unknown at this point. I do know that the PO only lives in my state for half the year, so the van has sat most winters of its entire life.
That said, the van has been parked since last August, and there are no apparent leaks of any sort under where the van has been.

The issue I'm running into is the the brake pedal will sink to the floor when driving around. At a rate of maybe a 1/2" per second.
If I lift up and start to brake again the same result is had. There is no air escaping or "wooshing" sound that I've seen others reference when having the same issue.
The brakes do work well as the pedal is being pushed (as soon as the pedal is pushed the vehicle does start to slow down at a steady, what I would consider normal, rate)
Just to slow down from 60mph it may take 3 full "push until sinks to floor, lift foot, and push to floor" acts to make a complete stop.
I did not try fast stabbing of the brakes, the above statement is just normal/linear stopping pressure by my foot.
Van does not pull left or right when braking. There are no ABS / Brake lights, etc of any kind on.

I've read multiple threads about RABS and the Kelsey-Hayes RWAL Antilock Brakes system, but am unsure if that applies to the 2003 model year, as most of those threads are referencing much older vans.
Was this system used on the 2003 MY Vans? And if so does this sound like an issue with the ABS Valve? Are there any tests that isolate it down to just the ABS valve?

Does this sound like a simple master cylinder issue? If so, what are the steps one can take to confirm or rule out it being the master cylinder over other potential reasons?

Or does it sound like a booster issue? (This was another common theme in the threads I've read) and are there steps one can take to confirm or rule out it being the booster?

I really appreciate anyone whom takes the time to read and respond. The info out there for this via google search always end without a solution or fix.

Thank you

 

Since it has probably set a lot, I'd bet its a seal in the master cylinder. You could plug the holes for the lines and see if it still does it. That would eliminate the abs.

 

Sounds good, looks like I'll just start with a master cylinder and fresh fluid.

 

 

Would you think old fluid / air in the lines would cause a sinking pedal?

I've had air in the lines in previous vehicles and got a spongy pedal feel but never a slowly sinking to the floor type of feel.

It basically has a pedal feel that you get when you're helping someone bleed brakes amusingly enough.

 

 

Unbolt the master cylinder from the brake booster. Pull the master out a little and check for wetness. If there is brake fluid the master is bad.

 

Classic symptoms for a bad Master Cylinder. Be sure to bleed it on the bench before installing (and you'll still need to bleed it there as well)

 

Air in the lines usually shows up as a spongy pedal that firms up after pumping it a few times. The pedal slowly dropping is either bad master cylinder or a leak, so if you aren't losing fluid very quickly it's almost definitely the master cylinder.

 

It's most likely the master cylinder. I said how to confirm that in my last post.

But I've also had rear wheel cylinders that leaked and would act the same way. It was a small enough leak that it didn't leave any puddles. But it did show up as wetness on the back side of the rear rims/tires/backing plates.

 

^ this..not air in the lines

 

Thank you everyone for your help on what to look for, I've updated the first post with a couple new findings.

I'll be pulling the master off again to check for wetness.
With my luck I bench bled it poorly, or the master was bad out of the box new.

I look forward to everyone's insights on the new "findings"