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hello all,
I am about to order brake line for my truck and I wanted to ask for some advice before I make an expensive mistake. I have stock 58 NAPCO axles, with cross drilled drum brakes and a power dual master cylinder from Brothers. Here are my questions:
1. What size brake line should I use? The wheel cylinders and NAPCO rubber lines all have 3/16" connections. I read somewhere where the main lines are supposed to be larger than the rear axle line to build up pressure, can you shine some light on that?
2. Should I use a proportioning valve in the rear line?
3. The master cylinder has two different size exits. Is the front (closest to the booster) for the front wheels? What is the theory behind the two different sizes?
4. The NAPA guys who are going to make the flares said they can put a 3/16 nut on one side and a larger nut for the master cylinder. Any problems with that?

Thanks for any help. Attached is a diagram of the proposed brake system.
Thanks. Michael
Proposed brake system

Hy jomichael, do you know what the master cylinders original application is? If it is for disc/drum brakes, then you will have to install at least one 10 p.s.i. residual pressure valve in the line plumbed from the reservoir that was originally for disc brakes. If you are using drum brakes all around, then no proportioning valve should be required. I always assumed the difference in ports in the master cylinder was to make line cross connections impossible. You can buy port adapters to make your port connections the same. If 3/16" was your original size, there is no reason why you can't use it, hope that helps

Thanks 3B, I ordered the dual master cylinder from Brothers and when ordering they asked "disc/disc", "disc/drum" or "drum/drum". So I went for the "drum/drum", hopefully they sent me the right one! I wouldn't know, how can you tell?

Hy jomichael, you can't tell the difference from the outside, if you ordered a drum/drum master that is what you should have. Bench bleed it well before you install it, let us know how you make out

jomichael, I'm doing something similar to my 49 3100. I have the same question about front and back of the new dual master cylinder I installed. Since the mc is under the truck and facing backward, which end line goes to the front/back???

Generally (if not always?) the front brakes go to the end of the master cylinder farthest from the brake peal.

If you use the standard GM proportioning valve, visually divide it into two halves, the front brakes have two outlets from the valve, one at a 45 degree angle up, and one down, and the inlet for the front is on top close to these. The rear only has one outlet and it points straight back, the inlet for the rear in on top near this.

Grigg