juston.ledbetter Posted July 28, 2015 Share Posted July 28, 2015 sorry if this is in the wrong area. my problem is when I press the pedal the air drops from 120 to 90 with only one pedal push. then from 90 to 60 so on so forth.1998 mack front end trash truck Quote Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/ Share on other sites More sharing options...
Green Dash Posted July 28, 2015 Share Posted July 28, 2015 The most obvious cause for this would be a leak in the system. Can you hear any leaks with the engine off? If not you might need to find someone to work the brake pedal while you check around/under the truck. If it is a leak, it should be easy to find with such a large drop in pressure. 1 Quote Jim Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298218 Share on other sites More sharing options...
HeavyGunner Posted July 28, 2015 Share Posted July 28, 2015 Yeah what green dash said, it's most likely to be a leak. Have someone work the pedal for you and snoop around, could be a blown diaphragm in an air can. Either way the leak should be on the service side of your air system in case you need to start tracing the airlines. 1 Quote The problems we face today exist because the people who work for a living are outnumbered by the people who vote for a living.The government can only "give" someone what they first take from another. Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298220 Share on other sites More sharing options...
j hancock Posted July 29, 2015 Share Posted July 29, 2015 Leak.....BIG leak. 1 Quote JimIt doesn't cost anything to pay attention. Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298228 Share on other sites More sharing options...
scottwetzel Posted July 29, 2015 Share Posted July 29, 2015 Front brake chamber maybe since it only happens when pushing the pedal?? If not that like everyone else already stated it def a leak somewhere. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298241 Share on other sites More sharing options...
chief doug Posted July 29, 2015 Share Posted July 29, 2015 does the air continue to drop while on the brake. If not check all the tanks for water or oil. If there's oil or water in the tanks, it takes away from air capacity, literally makes it like the tanks are smaller. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298246 Share on other sites More sharing options...
juston.ledbetter Posted July 29, 2015 Author Share Posted July 29, 2015 thanks for the quick response. checked for leaks this morning. found one small one, but I don't think it is severe enough to cause such a drop in pressure. purged the tanks did find condensation with a little oil. but still doing the same thing. the air does not continue to drop while you are pressing the pedal. it drops as air moves from the tanks to the chambers. then holds steady. I did have someone work the pedal while I listened for air leaks. no chambers are leaking or valves that I can hear. Quote Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298307 Share on other sites More sharing options...
juston.ledbetter Posted July 29, 2015 Author Share Posted July 29, 2015 HeavyGunner, could you elaborate on the what, when, and where. of a bad diaphragm in a can. thank you Quote Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298308 Share on other sites More sharing options...
Freightrain Posted July 29, 2015 Share Posted July 29, 2015 The rubber diaphram that the air pushes to actuate the brakes. Here is a picture to show you the parts.I would guess that the truck has relay valves for the rear. Maybe an issue there? There has to be something that is dumping large volumes of air. How many air tanks on the truck? 2 Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298310 Share on other sites More sharing options...
HeavyGunner Posted July 29, 2015 Share Posted July 29, 2015 HeavyGunner, could you elaborate on the what, when, and where. of a bad diaphragm in a can. thank youWhat freight train posted above Quote The problems we face today exist because the people who work for a living are outnumbered by the people who vote for a living.The government can only "give" someone what they first take from another. Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298313 Share on other sites More sharing options...
juston.ledbetter Posted July 29, 2015 Author Share Posted July 29, 2015 wow great post thanks freight train. I has 2 tanks Quote Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298314 Share on other sites More sharing options...
HeavyGunner Posted July 29, 2015 Share Posted July 29, 2015 Or if you think the problem isn't a leak it could possibly be the gauge. Quote The problems we face today exist because the people who work for a living are outnumbered by the people who vote for a living.The government can only "give" someone what they first take from another. Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298317 Share on other sites More sharing options...
theakerstwo Posted August 1, 2015 Share Posted August 1, 2015 Don't forget not ever one knows how to adj brakes so if all brakes are backed off then it takes much air to fill the chambers. 1 Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/41127-air-pressure-droping-to-much-when-you-step-on-the-pedal/#findComment-298624 Share on other sites More sharing options...
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