Truck is 1992 E350 cube dual wheel cab and chassis 5.8L

Initial problem - Front brake lines rusted out, lost pressure, MC bled dry.

After replacing all front brake lines and bleeding front and rears, I got good solid
pedal with engine off, I mean I can push long and hard and it stays firm.

When I start engine, the pedal slowly goes to the floor with almost no resistance
and you hear a woosh from the booster until it gets to a few inches from the floor
and the pedal firms up and the brakes operate (stopping power).

Repeated pumping has NO effect on firmness nor does the pedal pump up or rise.

So I replaced the master cylinder with a NAPA part and bench bled it for a good 20 strokes until only fluid was recirculating.

Installed MC and then bled brakes again (full quart) until no air came from bleeders.

I still have the same problem.

The pedal is high and firm and stays firm when I stand on it with the engine off.

Start the engine and the pedal sinks nearly to the floor before it works.

 

vacuum leak.

 

because your brakes work correctly with the engine off, this seems to point to a problem with the booster. i once had one that would apply the brakes all by itself whenever the engine was running. i would start with a new brake booster and go from there

 

If it was a vacuum leak it would be hard to push with the motor running. Did you bleed the rabs? The little module on the drivers side the rear brake line comes out of. My guess is you still got air, a leak or the rear pads need to be adjusted.

 

I'll try bleeding the rabs but I don't get how it could have that much air in the system if
the pedal is high and firm and the brakes work fine with no boost. I confess I don't know
how the rabs system actually operates so I'll take your word.

Is it possible the atmospheric valve in the booster is not working therefore I'm getting an
over boost situation and that is causing the pedal to travel so far?

The booster has 220k miles on it and is only $100 so I don't mind trying it.

 

I don't know I just know I had the same problem with my F250 it took me forever to get all the air out. If the booster is bad and making it too easy to push take it for a drive and see if you have brakes with the pedal half down.

 

I drove it with and without vacuum boost.

With boost the pedal slowly sinks to 2" from the floor hissing the whole way then the
brakes work and the pedal doesn't go all the way to the floor. If I pump the brakes it
does not build pedal like it would if it had air in the system. It just keeps making the
hissing sound and returns to about 2" from the floor then works.

With vacuum removed the pedal is high and firm and the brakes operate normally but
with no power assist so of course you have to push really hard.

 

the hissing is a clear sign of a leak in the booster. as i said in my above post, i have seen booster failures apply the brakes by themselves, so not all leaks respond the same. looks like you're buying a booster tomorrow

 

I replaced the booster, no improvement.

Dumb question;

On the master cylinder which port feeds the front and rear brakes?

I'm wondering if maybe the two lines got crossed up.

It's a 92 E350

 

Still trying to fix my low pedal problem....

I read here that a bad RABS valve can cause low pedal.

It also says the ABS light doesn't always go on if the dump valve is leaking.

Kelsey-Hayes RWAL Antilock Brakes

 

On my 97(7.3PS) the RABS is bled via a Ford capable diagnostic computer. And you can bet there is air in the RABS if the master cylinder went dry.

Do you have a cover held on by a bail over your master cylinder, Covering both front brake and rear brake reservoirs?
If so, the front brakes are the larger of the two reservoirs.

 

Fords412,
On the RABS dump valve on driver's frame rail, there is a lerge bolt (1 1/4?), remove this bolt ( careful it is under tension from the spring). Remove the spring and replace the bolt ( you will not lose fluid). Bleed the system and you should have a good pedal. If you do, you have the option of leaving the spring out (it will revert to "regular" brakes, or replace the dump valve to restore ABS. The ABS light will not illuminate if you leave the spring out. Good luck,
Ed
ASE Certified Master Technician
ee35805@yahoo.com

 

My Nephew owns a 1983 bronco. We lost the brakes. We tried to bleed them. I am getting no fluid out of back lines?? Can somebody help??

 

open all the bleeder nipples and gravity bleed them...( means don't pump anything , remove lid from Master and keep adding fluid as needed and let gravity do the work )

 

Man I wish people would post what the fix was when they finally fixed it. I have been reading for days about my issue and I see others having the same problems but they never posted what their fix was. Sigh.