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Your brakes will be air over hydraulic. This means that air from the brake pedal valve will apply a hydraulic master cylinder which applies pressure to the wheel cylinders in the brake drums.
Having air pressure is essential of course, but you need a satisfactory hydraulic system for the brakes to work effectively.
It is possible to boil the brake fluid if you seriously overheat the brakes - especially if the brake fluid is 'old' (it attracts water vapour and it is the water that boils when the fluid gets hot).
This leaves 'air' in the system and the brake system needs bleeding (as a minimum) or, ideally, fresh brake fluid. It is recommended to change the brake fluid every few years (depending on the operating climate).
Alternatively it is possible to 'cook' a wheel cylinder seal if the brakes have been overheated (such as a long downhill descent under load) and subsequently the seal will leak fluid (contaminating the linings) or might 'suck in' air into the system - again requiring bleeding (as well as renewal of the faulty seal(s)!).
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