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I am working on a 1953 M37 that has been modified by the fire service and again by some bubba mechanics. I doubt I am going to try to fully return it to full military form but I do want to get it it good running order. Right now brakes are on the platter. This has had a hydrovac system added to it. The truck sat for many years before I got it and the brake system had a lot of moisture in it and hence a lot of rusty parts. I am replacing all of the wheel cylinders, I have rebuilt the master cylinder and my plan is to just bypass the Hydro-vac and go back to stock brakes. When the hydro-vac was put in they did a pretty good job of plumbing it so that it did not disturb the stock brake system much at all. All they did was run the line from the stock master cylinder out the back of the cylinder and over the frame and then routed it to the hydro-vac mounted on the frame back by the rear. Then they ran the return pressure line back and over the farame by the master and tied it into the stock fitting where the original line would have gone to the frame. The line from the master as it sets is a 3/8" brake line leading to the Hydro-vac. This looks like it was the stock size since the return line that goes into the fittings on the frame is the same size. I just want to verify this so I can fab up a line from the master to the frame. The fitting going through the frame has three adapters on it stepping it up and then back down on the other side of the frame but this appears to have been there originally. The rest of the lines on the truck were replaced with a full M37 replacement kit. So, Bottom line, what is the stock size brake line out of the master cylinder to the frame connector?