Dug's 1959 Fleetside

[dsraven]

also check the ground circuits on all the sensors and trace a good ground back to the battery. I usually buff any paint off the ground surface of the engine and install a new ground strap there with a star washer to ensure connection. tighten everything up and coat the terminals/connection with some anticorrosion goop or paint. the same on the body and frame. these trailblazer/envoy harnesses have many ground connections through out the harness so ensure each one has a connection to ground. check the values at the connector plugs on the sensors as well. look for vac leaks on manifold and brake booster. since you were also having brake issues possibly related to the booster maybe good to simply disconnect and plug that vac line and see if that helps. remember you will have worse than manual brakes because the booster won't be assisting your pedal application and the bore size for the master cylinder is usually a big bigger for power brakes. also check the brake pedal and linkage connections to ensure the pedal isn't pulling outwards on the booster from the pedal side or slightly pushing on the booster from the weight of the pedal with no return spring. this may cause an internal leak inside the booster which is basically a vac leak. any wrong reading from the maf or mas will cause the fuel rate to be off which will mess with the o2 sensors downstream from that. if left too long the cats will get plugged up and cause back pressure and more problems. cat conv. are not cheap so best to get it right before too long.