Welcome to Karting!
To recap on fluids for anyone reading this in the future…
DOT 5.1 is compatible with DOT 3, 4 and 5.
DOT 5 is NOT compatible with 3 & 4.
It should be noted though that higher DOT numbers are not necessarily “better”, especially in karting where generally we don’t build a lot of heat in the system.
With a used kart I generally recommend that you test the fluid in the system to determine the type that’s in there using the water test as the previous owner may not in fact have used what was recommended by the manufacturer.
Freeline make a lot of (good quality) kart components from bodywork to brake systems, hubs, axles and much more. You will find these mostly on Birel (Now Birel-Art) chassis: Ricciardo, Kubica, LaCerc.
Since I’m on a bit of a freeline kick right now after digging around…here’s their brochure:
catalogo_freeline-min.pdf (5.0 MB)