Help with brakes

[Fear This: Cummins!!]

I have a a 1999 chevy Silverado 1500 4x4 with 99,000 miles. A brake line recently rusted through and I had to have all the lines replaced from the master cyclinder back. Upon picking up the truck, I noticed the brakes worked great, until the speedometer reaches 5 mph. At that point the pedal sinks and inch or so (does not bottom out) and then goes very firm and you loose most of the braking power or sometimes all the braking power (That will cause some grey hairs to pop out). Most of the time You can feel the pedal pulsate a little bit and it sounds like their is noise coming from underneath the hood. After looking at all the gunk in the master cylinder reservoir, I Put on a new master cylinder and cleaned up the reservoir. Still does the same exact thing at 5 mph only. Any help would be appreciated?

 

[PToombs]

Sounds like something may be wrong with the ABS, pedal pulsating and noise from under the hood. If someone else did the work, I'd get it right back there and have them make it right.

 

[Little Bill]

Damaged ABS wheel speed sensor, probably.