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While talking about the steel brake lines there are two points that should be brought up.

1-Which brake fluid do you intend to use? DOT3/4 or DOT5 (silicon). The military used the DOT 5 silicon. You can flush out all traces of DOT3 or 4 but you can never completely flush out all traces of DOT5 silicon. The two types are not compatible.
There are thousands of words written on the subject of which is the better fluid to use but you should do that research via google and read up on the various opinions before deciding which fluid you want to use. Each has it's claim to fame and each has it's bad points. The type of use and environment issues are what should be used to make the decision. On these web sites it just boils over as a argument about opinions. Anyway if you choose to go with DOT3 or 4 then you will need to replace all parts of the brake system.

2-You will need to apply a wee bit more pressure to the steel lines to determine their condition. That foot pedal when applied results in a lot more than 100 PSI.

Thanks Wes K
Did DOT 5 silicon exist back in 1953? And if so do all the parts need to run on DOT 5 silicon ? I'm running DOT 3 .

The Army converted everything they owned to DOT5 silicon in the 60/70's. All they had to do was flush the DOT3 with alcohol and load the DOT-5. As I said above it's a bit tougher to convert back to DOT-3 or -4. There's nothing in an M37 brake system that ever needed DOT-5. Someone in Congress had a cousin in the silicon business.

I'm running DOT 4 in all mine.

What it the difference between 3 and 4?
Phillip

DOT 4 is compatible with DOT3 and DOT3 systems. DOT4 is designed to absorb moisture slower than DOT3 and has a higher boiling point.