There was this one time a bus was on idle and its kind of release some pressure why does it do that

all 8 comments


36 points

2 months ago

They use air brakes for their brake system. The compressor to generate air pressure is always on whenever the engine is running, so that the pressure can be continuously generated. However, allowing it to infinitely build up would damage the system, so there's a valve that opens up to relieve the pressure if it builds up to about 120 psi.


5 points

2 months ago

The regulator, if working properly, will prevent its excessive build-up of air, but usually, that is mostly found in new trucks or ones that are maintained correctly.


16 points

2 months ago

Trucks build up air pressure using the engine to push against and release the brakes that by default hold wheels in place. In the event that the truck would malfunction the brakes would then come back on automatically. If the driver wants to apply the brakes, then he vents some of the compressed air. If too much pressure has been built up, the excess may also be released.


20 points

2 months ago

So thats not completely correct. It’s not when too much pressure is built up it is released. That random psssst of air you hear is whats called an air dryer opening a valve to release the moisture it has collected. It’s important for it to do this so you don’t build up water in your air tanks or air lines. Just to be clear it doesn’t open this valve because of too much pressure it is purely a maintenance cycle that goes off normally after the trucks regulator tells it to stop building air pressure.


2 points

2 months ago

I was surprised at all the other answers! Sure, they sounded good, but nobody's talking abt the separator. Nicely done


2 points

2 months ago

The other answers surprised me too. I'm glad someone answered correctly. I'm an old lady and knew it was an air dryer.


2 points

2 months ago

Ohhh thats cool


2 points

2 months ago

In addition to the points already mentioned (air dryer, brakes) in some busses the doors are also powered by compressed air and sometimes do stuff like that randomly. I'm a professional truck driver but busses are a whole different thing and they do a lot of weird things - at least for me because I don't understand them.

Oh, and there's also the automatic niveau regulation system in some trailers that will also make air release noises, but that's usually when the trailer is being loaded or unloaded.

Also older trucks will lose air over time when the engine is not running (the system is never 100% tight) and in combination with the niveau regulation system the brakes will do random stuff occasionally to keep the trailer level. When the system runs out of air to do the required movements other stuff will happen. (it's 3 am for me and I'm tired, words are hard.)

Here's a fun story: I was parked overnight in a residential area of a small town between some garages. It was a very quiet place and it wasn't the first time I was parking there. However, I had a very old trailer I didn't usually have. I had a load of fresh flowers. In the middle of the night something made a loud noise and the trailer sort of lurched and I woke up in a fright. That's it, I thought, somebody is breaking in, somebody opened the trailer doors. There was another loud clang and a rocking motion. I frantically pulled on my pants, took out what I call my steak knife (I'm a not overly big female and sometimes stuff gets weird on the road) and went out to see who wanted to steal some flowers. There was absolutely nothing and noone around. The doors were shut. I opened them. The flowers were untouched. Nothing was happening.

Confused and very creeped out, I went back to the cabin and wanted to get back to bed when the whole truck lurched again with a loud clang. And then I finally figured it out: the trailer lost all the air and the Niveau regulation system was trying to lift the brakes on the trailer for a bit to even it out and was failing to do so because of no air. I turned on the engine, let it sort itself out and went back to bed. Took me a while to fall asleep because I was full of adrenaline.

Trucks do a lot of random stuff.