Know Your Air Brakes
Page 53
If you've noticed an error in this article please click here to report it so we can fix it.
Part 7—The Relay Emergency Valve
regulating valve. Air will then flow past both regulating valve and non-return valve and will charge and balance the trailer reservoir to the tractor reservoir pressure. As long as the pressure is above 70 lb. the regulating valve will stay open and air is free to flow and retain balance.
Now to the emergency operation. In example, either normal uncoupling or failure of any part of the emergency line will bring the emergency valve into operation as follows:— as the line is broken, compressed air below the emergency diaphragm will escape via the break in the line, and as the pressure drops so will the non-return valve and the pressure-regulating valve close and hold the air in the trailer reservoir. The pressure then depresses the emergency diaphragm, closing the upper emergency valve and opening the lower valve to the brake cylinders.
With the passage now blocked at the supply valve and the upper emergency valve, the full trailer reservoir pressure can now pass through the valve and apply full brakes. The brakes will stay on until all pressure is released by "blowing" the trailer reservoir, or recoupling the emergency line. Recoupfing. has the eft-ea of placing pressure again under the emergency diaphragm, lifting the emer
gency valve and permitting the air trapped in the brake cylinders to flow to atmosphere via the exhaust side of the valve and at the same time charging and balancing the system.
A driver picking up a strange trailer can carry out a simple test to ensure that all is in order. Couple up, run the engine and fully charge the system, close the cut-out valve in the emergency line and then disconnect. This should bring the trailer brakes on instantly. Then check for any sound of leakage at the emergency valve exhaust, and on recoupling and opening the cut-out valve the trailer brakes should release at once.