AuroraGirl
I'm Awesome
You need to change the master cylinder to do this properly., the drum brake master cylinder in your truck powering the discs will cause them to drag. I dont know how GM did the residual pressure , iirc it was through the ABS unit (Someone like @Schurkey should be better suited to fact check that?), or in a valve on the masterI did the NBS rear disc brake upgrade. It was pretty easy. Find your self a complete 10 bolt rear axle with brakes from a 99-03 silverado/sierra 2001-2006 Tahoe/Suburban. By complete I mean get the parking brake cables, brakes...everything. You will be transferring the backing plates and brakes from it on to your rig. I bought one for $200. Took the needed parts, and re-sold the axle for $150. You will need the axle spacers for the backing plates. Cunningham machine makes them for both 10 and 14 bolt rear ends. I made a bracket to hold the parking brake cables to the frame, and had to buy the intermediate parking brake cable from a 2003 silverado. There are 3 sizes... Mine used the middle length. Took me about a day for the install. Not difficult. You gain better/smoother brakes that wont fade as bad like drums do, but the parking brake won't be as good. Drum brakes do make one hell of a good parking brake.
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The discs also need more volume than provided in the rear portion of a master thats disc/drum
Either drag or not enough take up would be what happen , assuming no changes are made