Those are not brakes,they are "retarders",which can be fitted in the drive-line,and are operated in conjunction with the main braking,usually pneumatic. They lighten the duty cycle on stop/start vehicles braking systems. A "souped up" version of this principle is used on electric vehicles and trains,to facilitate regenerative braking,where the energy dissapated in braking,is fed back into the batteries or power system. I have worked on electric braking systems on large yankee caravans,but because these operate on the bang-bang principle (on or off),the maximum braking applied has to be set way below any risk of locking up,and is therefore only an "assistance" to braking. Any proportional regulating of braking on this set-up would be quite complicated. I was surprised to see an electric braking set-up,on a mates recently purchased small tipping trailer,to go behind his quad. It also had an electric tipping ram,it looked trick,and i wired it all up for handlebar control,and gave it a year before it died...it lasted 3 months...Chinese r&d obviously not including British horse wee
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