1950 Ford Custom Brake Master Cylinder

[Flathead Fever]

I would definitely flush all the old brake fluid out. On a vehicle that is driven regularly you are supposed to replace the fluid every two-years because the brake fluid attracts moisture. I was a fleet mechanic for 30-years. What we did was drain the master cylinder and fill it with new fluid. Then as we were changing the brakes we would open up the cylinder on that wheel and let the old fluid gravity drain out while we were changing the lining. I would look at the rear of that master cylinder and make sure it's not leaking from the rear seal. It's normal for the brake fluid to squirt up as the pedal returns, that's just the fluid returning back to the resevoir, If you have a bad gasket it can slosh out and empty your resevoir You want to really keep an eye on single systems, you hace no emergency secondary resevoir to stop it with like modern cars There's nothing wrong with a single systems "if" they are maintain.