So you have a Wilwood reservoir type master cylinder with what I am pretty certain is an SKM/ Righetti Ridolfi 4 pot spring return caliper.
Bleed as suggested by Bryan Hall above. One side at a time. Fluid is likely to be normal DOT4 glycol but you can check using existing fluid. Should dissolve in water.
Check that the master cylinder piston is returning fully or it will not pump.
Pics of caliper and pads at kartstore .co.uk/brakes/ Righetti/SKM.
The caliper is ‘European’ if I have it right.
Have you found any identification numbers etc. on the chassis? Common places:rear cross tube, rear axle bearing hangers, bottom steering column bracket.
Edit. Your link suggests that your m/cyl is only 0.5 inch bore. If so it is very small for that caliper.