I'm driving a city owned truck, 96 F350 utility bed. New job.

The brakes work well enough, but the pedal will go all the way to the floor if I keep pushing. The (city) mechanic replaced the master cylinder and the fluid is brand spanking new, no real change. They shrug their shoulders and tell me "it stops, what's the problem?"

The problem is that I have a commercial license and the law states I'm responsible for an accident even if the garage certifies the truck. Not to mention I don't want to hurt anyone.

Anyway, I was wondering if this could be an issue with the anti-lock system, since no fluid loss is happening? Any other bright ideas?

 

I've seen a couple rabs valves do that. To diagnose them I had someone push the pedal (engine off) while I listened to the rabs valve with stethescope. You can use a long screwdriver or something held against your ear and the valve. I could hear the spring inside creaking as the fluid leaked into the chamber. You can also bypass the valve but it has some oddball fittings on the lines.

 

buddys gmc sonoma had that and it turned out to be a front wheel bearing wiped out and causeing the caliper to move weird. found that after he replaced the master and booster.

 

Could be air in the system. Ask the mechanic if he bled the RABS module on the left side frame rail.
Could also be a bad RABS module as Bashby says.

 

After the mechanic replaced the master cylinder did he adjust the rod coming out of the brake booster to load the M/C properly. I have replaced a copule of these now and if there isn't a bit of preload on the master cylinder the pedal will drop. Mine was that way until I made the adjustment. There is a thread on the forum about just that. If you pull the M/C off you will see the push rod with a threaded end on it. That's what needs adjusted to load the M/C.

 

Sorry for the delay in responding folks, I'm moving.

Thanks for the tips, now we'll see if the city mechanic is up to checking it out.