I have 4 lights on
anti lock
service vehicle soon
brake light
trac off
went to sun auto and they said that it would cost about $800 to fix. At that point I freaked and dont remember exactly what they were going to do. I believe that there is a code 20 and 24 or 26 they want to replace the abs valve does this sound right? Or is there an anti lock module?
Is this something easy to replace by myself?

no code 20, the blink system can't read zeros.

24 - RR wheel speed = 0 MPH
26 - RF wheel speed acceleration fault

are you sure those are the right codes? both of those deal with the sensors, not the master cylinder. go to a buick/GM dealership if you can for a senond opition.