Here are the most likely causes:
* Brake Line Leak: This is the most common cause. A leak in any part of the brake lines (metal or rubber) will allow air to enter the system, causing a hissing sound and potentially leading to brake failure.
* Wheel Cylinder Leak: Located at each wheel, these cylinders push the brake pads against the rotors. A leak in a wheel cylinder will cause a hiss and spongy brakes.
* Caliper Leak: Similar to wheel cylinders, leaks in the calipers can lead to hissing and brake failure.
* Master Cylinder Leak: This is the main component that distributes brake fluid. A leak here is extremely serious and requires immediate attention.
* ABS System Leak: Less common, but possible. A leak within the anti-lock brake system components could also cause a hissing sound.
What to do:
1. Do not drive the vehicle. A brake leak is a significant safety hazard. Driving with a brake leak could lead to complete brake failure.
2. Have the vehicle towed to a qualified mechanic or repair shop. Describe the problem clearly – hissing sound when brakes are applied.
3. Get a thorough inspection. The mechanic needs to identify the source of the leak and repair it properly. This will likely involve replacing damaged lines, cylinders, or calipers. They'll also need to flush and bleed the brake system to remove any air.
Ignoring a brake hiss could have serious consequences, leading to accidents and injuries. Prioritize getting this fixed immediately.
