The photo you show the rusted cylinder on is due to rust outside not internal so brake fluid won’t stop that. If it does you have leaks!
As you are doing a rebuild then now is the time to consider upgrades. That way you are only replacing old parts with upgrades so not much more £.
Braided pipes is an obvious easy one. Invest in a brake bleeder kit so you can do one man fluid changes easier either annually or every 2 years. No need to have a pedal pumper then! Dot4 fluid is not expensive so you can get fresh every time. The brake system is reasonably airtight so no big exposure to moist air. It can become part of routine checks/maintenance.
