What can cause the gas pedal to sink uselessly to the floor?

The brake pedal will sink to the floor when applied and the brake fluid reservoir remains full.

You don't have brake fluid There could be air in the system and it requires bleeding to rid it of air. If the vehicle is an older one, the master cylinder may be porus or the bore pitted. It should be resleeved or replaced. The seals could also be flat. Jam your foot down hard on the brake pedal. Is the pedal high? Does it sink to the floor if you rest your foot lightly on the pedal? www.powerbrakeandsteering.co.za

You will be adding brake fluid to the reservoir often.The brake pedal won't feel firm.The brake pedal may sink to the floor as you are applying steady pressure.Do a visual inspection of the brake lines looking for that wet spot.

The ocean floor doesn't sink from just age alone. As time passes, the plates under the ocean floor continue to move, and eventually they sink under the Earth's mantle in the process of subduction.

First, make sure the master cylinder is filled with brake fluid. If it is low, there may be a leak somewhere in the brake lines or a brake cylinder. If the master cylinder is bad, you can normally pump the brakes up, but the pedal will slowly sink to the floor again. The master cylinder needs to be replaced or repaired.