I need to add brake fluid to a 1929 ford pickup. I don't know if it has dot 3 or dot 5 brake fluid. Is there some kind of test I can do the will tell me what is in the pickup. How do I tell if a hot rod has dot 3 or dot 5 brake fluid?
Basically, it won't have DOT5 brake fluid. DOT5 is silicon based fluid. The US Military uses it extensively, but beyond that, it's not used very much. This means there's about a 99.9+% probability it has DOT3 or DOT4 fluid in it (the most common).
Instead of worrying about which fluid is in it, flush the old fluid using DOT4 fluid and call it a day. If this hasn't been done in a while (past two years), flushing it is a good idea anyways. This will get rid of any old fluid (no matter what is in it now) and get your brake fluid back up to snuff.
Paulster2 is absolutely correct, but I would only add that typical "hot rod" Dot 5 usually comes in funky colors (eg purple, green, dusk melon with a hint of fushia...)
The reasoning behind that is exactly your predicament.
Nevertheless, Paulster is again correct [sigh] that regular flush (especially on a vehicle new to you) is far more valuable than trying to match old brake fluid.
To me, it's like the century-long never-dies eternal debate over expensive synthetic engine oil. The stuff gets dirty no matter how great it is, whether it was hand-squozed from Siberian Fire Ant glands, and no matter who makes it or how much it costs. Just like brake fluid. So I say use name-brand conventional dino juice in big jugs from places like Walmart, at which point much invective and mouth foam is thrown about.
I digress. As I often do. It reminds me of that one time that ...
Flush a full container of DOT 3 or 4 through your system. You'll be glad you did. I personally love FoMoCo DOT 3, the blue can (plastic now) which has a higher dry and wet boiling point than DOT 4 and most "race" fluid. I'm not sponsored and YMMV.