Dear children,
For some time I have been curious as to why the following occurs in fire appliances…
When they always make a noise when the driver releases the brake pedal. It can be seen at 0.15 and 0.25.
It’s almost like your in a normal cab and you put a lot of pressure on the brake like your trying to drop air, but fire appliances always seem to do it each time the driver touches the brake.
Any idea why this is so?
Quick Release Valves
Oh. Thought this was gonna be about T&Cs at the catering warehouse firm. 
Haha no no.
So what is the purpose of the quick release valve? Why is it only fire appliances that tend to have it?
Do the compressors work at a higher rpm or something?
When you back off the brake, the quick release valve shoves the brakes off, to make them, errr, release quick.
Tractors, trailers, rigids all have them.
Without a quick release valve your footbrake would react like your park brake, it would take a second to release.
They’re not unique to fire engines.
Why can you hear this one? I dunno.
The reason the release of air is more audible on this video is because Fire Service drivers tend to apply the brakes hard, so when they release the foot brake some of that extra pressure is released so more noise is made. It would do the same on any other vehicle but normally haulage drivers don’t apply the brakes with the same effort, otherwise whatever might be on the back will move, a lot.
Fire Appliances have every piece of kit in the lockers either fastened down or secured in some way so with hard braking or cornering nothing moves. It isn’t the type of driving you would encounter on a normal HGV course, these vehicles are worked hard when needed, they might not do the mileage of their haulage contemparies (the chassis are normal haulage types nothing is done to make them special to their role) but they are inspected/serviced to a higher and more frequent requirement because of the way in which they work, or should I say are worked!
All also have powerful retarders, exhaust brakes are next to useless for this work so are usually disconnected. So to answer the question simply, the driver isn’t so much evenly applying the brakes but rather heavily applying them, not so much so during normal driving but during Blue and Two’s as in this case. By the way he made a balls up of the reverse but that’s par for the course too! Cheers Franky.
They make a lot of noise because they dont have valve silencers fitted as most modern trucks do.Also trucks dont use quick release valves they are fed by close mounted relay valves one per axle.The bottom of the relay is fed with tank pressure the top is linked to the foot valve.When you apply the brakes a small signal is fed to the top of the relay diaphragm which allows air from the axle tanks to pass to the chambers and apply the brakes.This is because you can move a small amount of signal air much faster than a large amount of chamber air and apply the brakes(and release them) much quicker by not having to pass the air that applies the brakes up to the cab and through the foot valve.The relays work like a transistor small signal ,large output.
QR valves on trailers are used on the park circuit not on the service line circuit as the service circuit has to exhaust through the abs modulator block or the abs wouldnt work as they do not have line modulators unlike trucks.
Qr valves are just a feed from the relay valve,a diaphragm and either 2 or 4 outlets.When air passes down the feed the diaphragm blocks the exhaust port and feeds air into the spring brakes to release the park brake.When the feed is dumped via the park valve or pulling the red line the exhaust port can open and the spring brakes can vent to atmosphere and the springs apply the brakes.
Thought it was gonna be about urinating on them to unfreeze them. 