Why airbrakes?

Hydraulic brakes were around before power boosters. Then again, 100 years ago the brakes weren't even at the wheels. You had bands in the transmission that slowed you down. Drop a drive shaft heading down a hill and you were off to the races...

Brake systems certainly have made some progress over the years, but the simple fact that if a line breaks you have no brakes is one of the many reasons I prefer manual transmissions in my smaller vehicles as well. Downshifting slows you down decent enough as long as you were anticipating the need to stop...then use the parking brake (or just shut the vehicle off in gear) once you're just about stopped until you're ready to take off again. In an automatic, lose the brakes and it's a little trickier...but not completely unmanageable as long as your vehicle isn't so new that the computer won't let you shift to neutral or so old that the parking brake cables are seized up or broken...I wouldn't buy a vehicle without mechanical linkages between my hand and the transmission and a key or other switch to kill the ignition system, as these new computer controls remove the options one might otherwise have available in the event of a malfunction.

 

Now go out and drive an old truck with mechanical brakes. Good luck stopping that in a reasonable distance.

 

There are a few semi trailers out there with hydraulic brakes.
They use an air activated hydraulic master cylinder.

 

They also made a vacuum over hydraulic which sucked.

 

That is a English Vacuum correct? At least what their trains used versus our air use.

 

You know I can't be real sure. I used to have a goose neck years ago that had them. The vacuum worked the master cylinder that was mounted on the trailer. I just used it for hay around the farm. Got a deal on a 48 spread that eliminated it's need LMAO

 

Awesome.

 

Air safety brakes were invented by George Westinghouse in the 1800's to prevent runaway/roll away trains. Other advantages included ease of coupling/uncoupling, and the fact that leaking hydraulic fluid cannot be replaced on the go but air can.

 

Fluids are non-compressible.
Pedigreed Bulldog has it right. your trailer would have to stay hooked all the time. You would need a big hydraulic fluid tank, and fluids do burn when they are overheated. Air systems are lighter, cleaner and have minimal maintenance when compared to hydraulic brakes.
Of course low air and dragging brakes burn up all the time, when the warning systems are faulty or ignored.