Air Line replacement

Color is your preference, me personally, I'd go with the correct colors just to make things easier in the future if there's a problem. One of my first jobs, I help plumb an old KW. I've seen all the charts and had brake training, but doing that really gave me a much better understanding of the dual brake system. To this day, I still think about those orange and green lines when I have an air leak on only 1 side.

Brass with ferules is hard to beat for reliability, but really suck to put on something like a foot valve, or in any tight places. Push-ins are easier and can be just fine as long as you get quality ones. I haven't had any issues with Parker brand. They have a plastic body, but the push-in parts are brass. Most of the one ones that have a plastic release collar, either end up leaking, or I can not get them to seal again once I take them apart.

This is what I use, and where I get them:
https://shop.tifco.com/catalog/category/id/4.64.1650

 

The OD is irrelevant it is the ID that matters. Even if the hose has a fitting at the end with a smaller ID do not reduce the hose ID. The reduced ID of a hose of over several feet can reduce the air flow more than a restricted fitting. Changing the hose sizes changes the time required for brake application and release.
Replace sizes as original. Keep both sides the same, 90's, 45's, and straights.
I prefer to use rubber between the frame and axles. Plastic for everywhere else, it does an excellent job and is inexpensive. I am also old school compression fittings but that was some interesting info on push in's from GrapeApe.

 

Our newer kenworth factory push pull fitting stink. I have changed more fittings on them and they all seep. My 2001 I have only changed 1 brass fitting in the last year. Probly 4 each per year on the newer kenworth.

 

On my Pete the leveling valve is fed independently from the passenger side air tank. There is a check valve there that opens at 45lbs? And closes at 65? Cant remember the exact numbers. It prevents what you're worried about. A good parts guy will explain it better.

 

I completely agree with this post. Haven't personally used the Parker brand, but after looking at them they do appear to be a happy medium between user friendly push clip and good old reliable brass.