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I was having a discussion today, which led to the question

why do heavy trucks still use air brakes?

To my knowledge, it has been used for at least 40 years (I remember that as a kid), and apparently (I was told today but I haven't gotten around to verify it) they are still widely used.

From what I remember, one of the things I was cautioned was that if you repeatedly pressed on the air brakes, then after a while the air buffer would empty and it would take time to fill up (thus losing braking capacity).

Anyway, I wanted to know what are the benefits and disadvantages of air brakes compared to other technologies, e.g. hydraulic lines, or electrical system, (even KERS systems for more modern electrical vehicles).

UPDATE: From the answers I understand that the main issue is reliability and "technical debt". I want to push a bit further and understand, what's stopping air brakes from being used in other vehicles. E.g. is it cost, performance, inability to accompany AC/DC drum rhythm?

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  • $\begingroup$ Per Solar Mike's answer, there's no reason to exclude disk brakes from an air-actuated system. The air-actuated portion moves a lever which would require a re-design of the mechanism if used on disk brakes, but you'll discover that trailers have drum brakes. Kinetic Recovery System (KERS) would add increased cost, complexity and weight to a trailer. $\endgroup$
    – fred_dot_u
    Sep 3 at 13:26
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    $\begingroup$ Whoever told you about emptying the buffer got it backwards: air brakes use air to release the brakes, not apply them. If the air system drains, you aren't going anywhere. $\endgroup$
    – Mark
    Sep 3 at 20:37
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    $\begingroup$ The fail-safe design for air brakes is very inherent to them, and was really the enabling technology for longer trains, which before air brakes had a brakeman who ran along the tops of the wagons to crank a wheel, in response to signals from the engine's whistle. One of many yesteryear occupations which today seems scarcely believable. $\endgroup$
    – CCTO
    Sep 3 at 21:16
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    $\begingroup$ @Mark: Train air brakes use air to release the brakes (this is partly because the need to connect the brake hoses from car to car greatly increases the possibility of a brakepipe rupture - for instance, in the event of a train separation - making it much more important that the brakes fail safe than would be the case otherwise). Truck air brakes (and, for that matter, a locomotive's independent brake) are simpler, and use air to apply the brakes. (At least, that's what I remember from reading NTSB highway accident reports). $\endgroup$
    – Vikki
    Sep 5 at 2:08
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    $\begingroup$ @Vikki - That spring system is what I understand the parking brake to be - if the pressure drops too low, then the brakes automatically engage from spring force. I regularly see skid marks on the highway that look like they are from just a trailer, not the tractor bit, and figured that that was what had happened - a hose sprung a leak and the brakes engaged. Although, the actual brakes can stop you faster than the emergency brake, since they can squeeze harder. See: web.archive.org/web/20210907222949/https://… $\endgroup$
    – IronEagle
    Sep 7 at 22:30
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Biggest advantage is that when connecting to trailers, there are no bleeding issues.

Imagine having to connect hydraulic pipes and remove the air bubbles...

As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out.

Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC.

But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with air brakes, they lock on as the air pressure is used to hold the brake shoes out of contact.

This explains some of those brake marks seen on motorways, as the trailer brakes lock on at 60mph when a pipe or connector fails, with the tractor unit still pulling.

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  • $\begingroup$ There are "self-contained" remotely actuated trailer brake systems, such as those found on recreational travel trailers. The pulling vehicle has an actuator that converts hydraulic pressure (or equivalent electrical signal for brake-by-wire) to compatible electrical signal which is transmitted to the trailer. This would require semi-trailers to have more complex braking systems than simple air-actuated systems and provide no benefit. $\endgroup$
    – fred_dot_u
    Sep 3 at 13:23
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    $\begingroup$ Air brake systems have a spring actuated emergency brake on the rears. The air pressure works against the spring releasing the emergency brake. When air pressure is lost the emergency brake is applied. The application of emergency brakes on a School Bus, travelling 55-MPH, causes the bus to safely but quickly roll to a stop. On ice all bets are off. $\endgroup$
    – Jim Clark
    Sep 3 at 18:33
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    $\begingroup$ There's no reason you can't design hydraulic brakes the same way - ie: loss of pressure = brakes ON instead of OFF. $\endgroup$
    – J...
    Sep 4 at 12:59
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    $\begingroup$ @J... must be, otherwise they would have done it. Most likely all the existing equipment… $\endgroup$
    – Solar Mike
    Sep 4 at 14:19
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    $\begingroup$ @SolarMike Yes, I mean only that the other reasons in this answer are more likely to be the decisive ones. $\endgroup$
    – J...
    Sep 4 at 14:24
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Several characteristics led to the adoption of air brakes. The primary one is reliability. A leak on a hydraulic brake system means the brakes don't work. A leak on air brake system means the compressor needs to run more. This is made more important by the need to apply brakes on a trailer, which needs to be connected to the cab easily.

The next is that even if a better alternative were found, there is already a millions-strong fleet of trailers that would require either retrofitting or having dual systems on power units. The only driver of this would be regulatory (a government mandate), and there is not a perceived safety benefit for such a change.

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    $\begingroup$ "A leak on a hydraulic brake system means the brakes don't work." And hydraulic shooting out in a high-pressure jet. $\endgroup$
    – RonJohn
    Sep 6 at 7:46
  • $\begingroup$ Hydraulic brakes with spring-loaded calipers work perfectly fine after a leak. It's the same concept as many air brake systems. Also no hydraulic jet shooting out as long as you have a flow fuse valve installed. sunhydraulics.com/models/cartridges/specialty/flow-fuse $\endgroup$ Sep 9 at 18:15
  • $\begingroup$ @JeremyBeale When I say work I mean be able to both go and stop. You can't turn on a compressor and get more DOT-5 fluid from the air. $\endgroup$
    – Tiger Guy
    Sep 9 at 18:41
  • $\begingroup$ That's the other benefit of a flow fuse valve. Your leak will last a total of about 2 ms. Plenty of hydraulic fluid left in your system when you power up again. $\endgroup$ Sep 9 at 18:44
  • $\begingroup$ @JeremyBeale I assume the flow fuse leads to a locked wheel? $\endgroup$
    – Tiger Guy
    Sep 9 at 21:04
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"Power" brakes, like the ones you have in your car, use air pressure to assist your pedal effort. In passenger cars the air pressure is actually negative: it relies on intake manifold vacuum. That vacuum is limited to about -15 PSIG: once all the air is drawn from a container, the difference between that pressure and the sea level atmosphere is all you can get. Air brakes use compressors to get unlimited (in principle) pressure which is needed in heavy truck brakes.

Brake geometry, disc/drum, has nothing to do with it.

The vacuum reservoir (I hate using that term to describe a continer of emptiness.) can be depleted by pumping the brakes while the engine is idling and that will increase the pedal effort - just like in air brakes.

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  • $\begingroup$ Aha. I wondered about the change in pedal feel when pumping the brakes in a stopped car. Never knew that there was a vacuum system in addition to the hydraulics. $\endgroup$
    – hobbs
    Sep 4 at 5:56
  • $\begingroup$ I worked on the railway and was discussing the vacuum brake system used on an old locomotive we were working on and the guy explained to me that the "exhauster (vacuum pump) fills the reservoir tank with vacuum which releases the brakes and when the driver pulls the brake lever it lets the vacuum out." I guess that if you over-filled the reservoir with vacuum that it would implode. $\endgroup$
    – Transistor
    Sep 4 at 9:34
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    $\begingroup$ The hard limit on the available pressure differential is also the reason why vacuum brakes are now obsolete on trains (even in Europe, where the railways stubbornly refused to adopt the Westinghouse air brake for decades after it became ubiquitous stateside). $\endgroup$
    – Vikki
    Sep 5 at 17:50
  • $\begingroup$ @Transistor The reservoir can't be over-filled. (For vacuum that would be over-emptied.) There's no PSIG below 0. I don't know why there would ever be a vacuum pump. There are obvious advantages to a pump for pressures above atmospheric. I vaguely remember (1964) reading an article describing difficulties in designing vacuum-assisted brakes. The main problem was that applying brakes would abruptly zero out the vacuum and there would be nothing to draw in fuel. I wonder now what vacuum they meant. Diesel and steam engines don't have convenient sources of vacuum. $\endgroup$
    – stretch
    Sep 6 at 1:39
  • $\begingroup$ of course in a diesel-engine vehicle we need a vacuum pump to do the same thing. $\endgroup$
    – Tiger Guy
    Sep 9 at 18:44

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