Brakes: rear locks up, pedal goes to floor

[Jethro Bodine]

Here are the symptoms. With the truck on or off, the pedal if held will creep all the way to the floor. If you slam on the brakes or even use them hard, the back end locks up. The ABS light stays on constantly.

I took it in and they changed the wore out rear brakes with a new hardware kit and all. (it was previously not holding well and making a noise). They also changed the master cylinder.

The back brakes work great now, only they'll lock up easy. I also had them bleed the brakes and they changed out the mast cyl twice since it still did the same deal with the pedal going slowly to the floor. They bled it at the ABS module also. On the second try, they found that the proportioning valve (looks like a big fitting between the brake line and mast cyl) has a broken piece of plastic and needs to be changed.

Where can I get this thing and also will this fix the spongy pedal (I dont see how) or is that something else?

[96F350KID]

The abs module is more then likely your issue fab up a by pass tube and see if it fixes your issue