Brake pedal fades

[rusty101_1999]

I'm working on a 1992 F350 7.3l 2WD Dually and cannot get the brake
pedal from sinking to the ground. When the trucks not running it takes a while for it to sink, but when running it has no porblems sinking to the floor.

I've tried 4 master cylinders, 2 proportioning valves, used RABS unit,
replaced front hoses & calipers and bled about 15 times, but to no avail.

When I clamped off the old front brakehoses the pedal would get & stay hard, so I thought it was calipers and it needed hoses because they were cracked anyways, so I replaced it but no avail. [img]/ubbthreads/images/graemlins/shrug.gif[/img]

Any suggestions to the problem?

[yaric008]

the booster maybe?

why did you go through 4 master cylinders?

[khuseby]

Just went thru a similar episode w/ my PU, sinking pedal, replaced the vacum pump, pedal still sinking, [img]/ubbthreads/images/graemlins/mad.gif[/img] then replaced the power brake booster now the pedal is rock solid. [img]/ubbthreads/images/graemlins/smokin.gif[/img]
Hope this helps

[rusty101_1999]

I soyuld've worded that better, the guy has had 2 master cylinders replaced in the past few years before i've seen the truck. I tried a 2nd one just in case one was a dud...never know.

Booster is ok, tried a new one also and didn't solve anything.