Why could you not use a gas in the brake pipes?

Answer 1

Theoretically, you could, but it would have to be highly compressed, and then, the pedal might be too hard to press (without making things complex and too bulky for a regular car). Let's start with some background. Matter takes three forms - solid, liquid, and gas. A solid has a definite volume and shape. A liquid has a definite volume, but can take the shape of its container. A gas has an indefinite volume and shape, meaning you can force nearly any amount into a container - assuming the container is strong enough.

Anyway, a liquid is the best thing to use in brake lines because liquid has a definite volume and responds immediately to any pressure behind it.. A gas would be too "spongy" (compression, motion loss) and not respond fast enough (too much latency). If the line is not pressurized to at least what is needed for braking, the shoes or calipers would not work. But if you have it too strong, the driver could not apply the brakes. So for general use in small passenger vehicles, fluid brakes are the most practical.

In trucks which use air brakes, there are a couple of ways they overcome these problems. They use both regular air brakes and "spring brakes" (for emergencies and perhaps for use during coupling). The two work in opposite ways. The regular airbrakes require air to release. While that can cause difficulties in getting the rig moving on occasion, the braking response is very good. That way, the latency and loss of motion introduced by the air doesn't interfere with braking. However, if there is a leak in the lines, the trailer will cause the rig to lock down, since the brakes are applied by default.

The spring brakes are not as good in braking as the main brakes because of the air latency and motion loss issues due to the nature of air. But that dilemma is reduced because of an air tank under the trailer and a relay valve. The tank addresses the challenges by putting the air source closer to the brakes and reducing the compression in the lines. I think it is a drop in pressure that causes the relay valve to engage (much like how the regular brakes get engaged). If I remember right, if you have no trailer, you have to engage the trailer brake valve to avoid a loss of air from the compressor.