Repeated use of the brakes in traffic and especially downhill causes heat to build in the brakes, but without the cooling effect that would normally occur at speed this heat soak can cause brake fade. Changing the fluid is a sensible step but if this has not solved the issue then there is clearly another issue. It is possible that you are experiencing too much contact of the pads on the discs, whether they are incorrectly set or the caliper is not operating as smoothly as it should, and this residual drag of pads on disc is causing the excessive heat and fade. Another possibility is that there is a small leak within the master cylinder - you can test this by starting the car on a flat surface and holding the brake pedal down with firm pressure for 30 seconds. If the pedal is firm initially but after a period begins to sink to the floor, this would suggest a leak in the system.