spamfree4 Posted September 5, 2022 Share Posted September 5, 2022 I have a 1970 DM685 dump truck which seems to run through it's air much faster than a modern truck. With the engine off and air pressure sitting at 130 PSI I can make 4 full pressure brake applications before the pressure drops to 65 PSI and the air warning turns on. There are 2 tanks in the system and all 4 segments drop to the same pressure, so I know it's using the full reservoir capacity. No leaks in the system, truck stops as it should when the brakes are used, no air leaking when the brakes are fully applied. Pressure builds from 0 to 130 in under 3 minutes when cold started. Total of 6 air chambers, not including the 2 parking / spring brakes. Is this just normal for a truck of this era? Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted September 5, 2022 Share Posted September 5, 2022 make sure the brakes are adjusted up. Quote glenn akers Link to comment Share on other sites More sharing options...
spamfree4 Posted September 5, 2022 Author Share Posted September 5, 2022 First thing I did. Front brakes had excessive slack, but after adjustment to minimum, no change. The rears are so tight you can barely see any motion when the brakes are applied. Parking spring brakes work as they should. Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted September 5, 2022 Share Posted September 5, 2022 is there a lot of water in the tanks, that is taking up room for air? and to the best of my memory a typical brake application uses 15-20 PSI. so 4 applications will be 60-80 PSI of usage.. are all of the rear brake chamber's 30's? (30 cubic inches) ... jojo 2 Quote Link to comment Share on other sites More sharing options...
Popular Post Onyx610 Posted September 5, 2022 Popular Post Share Posted September 5, 2022 Have you listened for any big leaks when making a brake application? 2 1 Quote Link to comment Share on other sites More sharing options...
Popular Post mechohaulic Posted September 6, 2022 Popular Post Share Posted September 6, 2022 when you say " all 4 segments drop the same" , does this unit have dual air system ( red and green needle on gauge);i don't remember when that started. when dump body was installed, were the air tanks moved or replaced? the main wet tank has a check valve connecting other half of tank;; when gauges are at 130 if you open each drain on the tanks individually ; does each drain have full pressure of air coming out ? have seen the one way check limit air to other tank half. tank not having an external check has one internal , pain in a-- to change, AFTER draining tank remove fitting on end of tank flashlight can see the internal check, long extention and grease on the socket will remove the check. have also seen an improper discharge line from compressor become restricted from wrong line too close to high heat compressor air. also as you stated ;;yes older units weren't made to build air like todays iron ;; NOT counting fire apparatus ; they had a rapid build up system. 2 1 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted September 6, 2022 Share Posted September 6, 2022 do you have an air dryer that hasn't had the filter changed restricting air flow ??? being a '70 doesn't mean someone hasn't "modified" things through the yrs . has this issue been there or just become noticeable ?? 1 Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted September 6, 2022 Share Posted September 6, 2022 (edited) You can check your “check valves” by draining the wet tank. If primary and secondary hold they are good. If one of them drains down that one is bad. Edited September 6, 2022 by Onyx610 Spelling 1 Quote Link to comment Share on other sites More sharing options...
spamfree4 Posted September 6, 2022 Author Share Posted September 6, 2022 No air drier in the system. All tanks have been drained, no excess water in any of them. The wet tank section tends to accumulate oil, but the air builds from a cold start to full pressure within the specification that the Mack service manual from that era indicates as OK. Check valves in each tank seem to work as they should. My first thought was that someone plumbed the air exit to the wrong tank segment and the system was being fed from only one tank. Testing has shown that is not the case, as all 4 tank segments fall to the same pressure when brake applications are made. The system has 2 tanks, with 2 drains per tank. The truck has been seriously neglected by previous owner(s) and the air piping is quite a circus. Reading all the comments, I suspect it might just be normal or have oversized pots, since it stops with minimal brake application when loaded to the top with dirt. Makes for a real fun time when trying to make K turns on a grade... It's off road only at this point. Thanks for all the input. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted September 6, 2022 Share Posted September 6, 2022 thanks right back to you for the latest update.it does become frustrating when questions are answered by many on this forum and the "questioner" just vanishes from the planet, LOL . 2 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted September 6, 2022 Share Posted September 6, 2022 Updates are great... 1 1 Quote Link to comment Share on other sites More sharing options...
fjh Posted February 7, 2023 Share Posted February 7, 2023 Yup good on you for following up! Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted February 7, 2023 Share Posted February 7, 2023 10 minutes ago, fjh said: Yup good on you for following up! F, look at the date bro…🤭 Quote Link to comment Share on other sites More sharing options...
fjh Posted February 7, 2023 Share Posted February 7, 2023 Better late than never bud! Thanks 🙏 Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted February 7, 2023 Share Posted February 7, 2023 Ok I’ll go with that haha Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted February 7, 2023 Share Posted February 7, 2023 Im actually surprised at how few air brake questions this site gets. Quote Link to comment Share on other sites More sharing options...
JoeH Posted February 19, 2023 Share Posted February 19, 2023 He probably has big brakes. Our 1980 dm686sx has 18"? Brakes instead of the standard 16"? Brakes. Truck always had great stopping power. I wish more trucks had them. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted February 19, 2023 Share Posted February 19, 2023 larger brake shoes a positive on vehicles especially dumps . back to original question of air consumption per brake application; based on everything written , i would go with a third tank or larger if room is there. shouldn't have low air buzzer with normal stopping every time. did a reread . this air drop is with engine off not driving. its seems ok then. doesn't state air loss when running. have a great day😄 1 Quote Link to comment Share on other sites More sharing options...
mowerman Posted February 21, 2023 Share Posted February 21, 2023 Seems like more like 8 to 10 times before our alarm goes off but that’s not mashing it to the floor and yes, it’s an old thread Quote Link to comment Share on other sites More sharing options...
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