Q: What causes severe vibration when braking at high speeds or downhill?

What you are describing is a sign of warped brake rotors. When you press on your brake pedal, this activates a hydraulic pressure which forces small pistons inside your brake calipers to push the brake pads against the brake rotors which is what causes the stopping power you are familiar with when pressing your brake pedal. When this happens, an enormous amount of heat is generated due to the friction between the brake pads and rotors. The brake rotors eventually cool down and then heat up again the next time you press on the brakes. Over time this can cause them to warp slightly causing very small low spots on the surface of the brake rotor. When this happens and the brake pads are applied again to the rotors, this causes an uneven surface (on the rotor) to be applied to the flat surface brake pad, which can cause a vibration, shaking or very loud grinding sound.

This may also suggest that your brake pads are worn and due to be replaced or that you have a leak somewhere in your brake hydraulic system. The brake reservoir has a sensor inside that detects when the brake fluid reaches a certain point which generally indicates that it is time to replace the pads. As your pads wear thinner and thinner over time, the brake lines require more and more brake fluid to maintain the same amount of hydraulic pressure in the brake lines. As a result, the brake fluid level sensor will trigger a warning light indicating that the brake fluid level is low. I would recommend having an expert from YourMechanic come to your location to diagnose your braking system to determine what may be causing this light to come on.