I am going to swap out the rearend in my 03.
Disc over drums.
Do I need to do anything to the brake system for this swap.
Proportional valve?

 

Not if your swaping the whole rear. but the master cylinder is different between 4 wheel disc and a disc/drum set up.

 

There is a little piece called a residual pressure valve that keeps the pistons out so the brakes work right away. As the springs on the drum system are much stronger than the seal on the caliper, the system maintains more pressure for the drums. If you don't change the master cylinder, your brakes will be dragging all the time, performance and mileage suffer, and pad life will be short. Swap it out and life is good!

 

A quick follow up as I am looking to do the same thing, I have an '05 Silverado with 3.23 open rear end with Drums and I want to put my 3.42 G80 axle with Disc in its place.

Sounds like besides just the basics stuff I need/should also swap the master cyl and I would be golden? No other valves to swap as the master Cyl is what has the correct proportioning valve in it?

 

Awesome! I was wondering for a while what kept that extra pressure on the rear brakes since I want to swap mine to discs. I've already done hydroboost and used the master cylinder it came with from a disc brake truck....looks like I'm good to go!