edit: Solved. Thanks guys
In: Engineering
There are a number of different tssss that a truck or bus can make.
Usually if it is sitting there with the engine running or driving, you are hearing the air dryer purge.
There is an air compressor driven by the engine that pumps air into the air tanks, but first it usually goes through an air dryer that removes moisture from the air.
When the air tanks have enough air, the compressor keeps pumping a bit longer and this extra air pressure is then blown through the air dryer which opens a valve on the bottom to blow some of the water out that is has collected.
If you look under the truck when it is doing this, you might be able to see a wet spray of air going to the ground.
Another tssss is when the driver releases the brake pedal. The pedal applies air pressure to the brakes, so when the brakes are released the air pressure gets let out.
The biggest tssss is when the driver applies the park brake. The park brake works the opposite to the foot brake, it is applied by big springs and uses air to release it.
When the driver pulls the park brake, all the air from the park brake chambers is dumped and the big springs are allowed to tightly apply the brakes.
Because heavy vehicles use air for so many things, there are a lot of things that can go tssss.
There is the air being released from air suspension, like when a bus lowers itself at the bus stop.
There are air powered and air assisted clutches.
There are even air powered or assisted gear shifts.
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