mickwoodie Posted June 21, 2011 Share Posted June 21, 2011 Hey guys,ive got a late modle metro liner that seems to have abit of an air issue. when the park brake is released it drops the service brake air pressure. im thinking the only spot the two air systems cross over is in the priority valve in the bottom of the air drier? any ideas? Quote Link to comment https://www.bigmacktrucks.com/topic/19474-park-brake-using-service-brake-air/ Share on other sites More sharing options...
ThaddeusW Posted June 21, 2011 Share Posted June 21, 2011 Hey guys,ive got a late modle metro liner that seems to have abit of an air issue. when the park brake is released it drops the service brake air pressure. im thinking the only spot the two air systems cross over is in the priority valve in the bottom of the air drier? any ideas?The spring brakes are fed by both the primary and secondary air systems via a two way check valve. When you push the yellow knob in and your air pressure begins to drop, you have a leak somewhere in the system. The air dryer feeds both the primary air system which operates the brakes on the rear driving axles and the secondary air system feeds the steering axle(s). Both systems are isolated by check valves. Both systems also feed the spring brakes on the truck/tractor and the trailer emergency/supply (red) line.There could be a leak in one of your brake chambers, air lines to the brake chambers or your spring brake relay valve. Let the truck build air pressure, shut the engine off, block your wheels and release the brakes. Get under the rear of the truck and listen for leaks. Also a spray bottle with soapy water sprayed on the brake lines/components will reveal leaks as well, though, I suspect if you see a rapid and drastic drop in pressure you have a large leak and should be able to hear it. See this diagram for a better idea of how the brakes work. Follow the yellow lines, they are the spring brake lines. Its not guaranteed your truck is plumbed the same, things may be slightly different in Australia/NZ and between trucks in general (the diagram is also a bit dated). But that diagram is pretty much the foundation of any air brake system in North American designs. Quote -Thad What America needs is less bull and more Bulldog! Link to comment https://www.bigmacktrucks.com/topic/19474-park-brake-using-service-brake-air/#findComment-80046 Share on other sites More sharing options...
mowerman Posted June 21, 2011 Share Posted June 21, 2011 welcome aboard,,hope vyou solve your problem,,sounds like an easy fix...bob Quote Link to comment https://www.bigmacktrucks.com/topic/19474-park-brake-using-service-brake-air/#findComment-80050 Share on other sites More sharing options...
mickwoodie Posted June 21, 2011 Author Share Posted June 21, 2011 Thanks for the schmatic mate.dont think i explained the problem properly.when the park brake is released the service brake air system pressure drops off (using about 1/3 of the avalible pressure). the park pressure gauge does not drop away as if the system is feeding from the wrong tank.this dosent seem right to me?the brake system hasnt been worked on recently so theres no chance of any lines being crosed over.it could still be some simple fault, i havent worked on on highway trucks since my appretiship many years ago so could be overlooking something simple.cheers for your help. Quote Link to comment https://www.bigmacktrucks.com/topic/19474-park-brake-using-service-brake-air/#findComment-80056 Share on other sites More sharing options...
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