[Lippyp]

Standard mineral brake fluid is Hygroscopic (this means that it absorbs water) If the fluid has not been changed for a long time the fluid can become waterlogged and this can lead to the boiling point of the fluid reducing leading to increased fade under heavy/prolonged braking and can also attack any iron or steel components in the braking system. This happens more if the vehicle hasn't been used for a while so I would imagine that it is possible that you have rusted calipers.

It's not worth taking risks with brakes so I would consider replacing them. Also check the flexible hoses on the front, it is possible for them to weaken and balloon when you step on the brake so losing pressure to the calipers. You should be able to get any brake shop to check your rotors to see whether they are still in tolerance.