ABS pump / Z71 Tahoe issues

[Tom danger Collins]

Vehicle. 98 Z71 2 door tahoe sport / lift kit / 5.7 L

(This is my first post) The parking brake/brake problem light is on. The brake pedal is very spongy. When I try to put it into 4 HI or Low the tires bark every second or so but in 2 WD its fine other than the brake pedal. The guy at the brake place cannot bleed the air out, and we have been through 3 master cylinders. The last master cylinder is GM manuf. The brake repair guy said it needed an ABS pump. My questions are.....are the 4x4 issues related to the ABS system in the truck, or is it more likely to be an issue with the transfer case? How much trouble/cost in replacing an abs pump

 

[SunlitComet]

If you drive i 4wd on dry pavement your tires will bark or chirp. The brake pedal does go down quit a bit on the trucks. Do you have the same spongy feeling when vehicle is engine is not running and vacuum booster boost is exhausted. That specific light should only light when parking brake is engaged or there is a output pressure difference feeding the into the ABS brake system from the master cylinder. It is possible you may be experiencing an isolation or PWM valve that might be stuck in a position which is exposing a master cylinder pressure line to one of the internal accumulator creating the spongy effect. A properly equipped shop with a compatible scanner can run actuator tests to determine if a mechanical fault exists in the unit that is not picked up by the electronics diagnostic sub system.