Will Posted July 20, 2023 Share Posted July 20, 2023 gday guys will here 1998 mack trident, i have full air and push my yellow button in and it leakes out of the exhaust part of the rear valve that services the rear boosters. i push the brake peddle in and the leak goes away and air builds up to normal again. i have pulled the service linesof each booster and no air is coming out when maxis are released so boosters are good. any idea where i should start looking now? thankyou will Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted July 20, 2023 Share Posted July 20, 2023 If you tap on the brake valve does it affect the leak? If so, I would say the valve is hung up with dirt or possible split o-rings inside on the piston.. Quote Link to comment Share on other sites More sharing options...
Will Posted July 20, 2023 Author Share Posted July 20, 2023 I’ve replaced the valve and it’s still doing it mate. I can’t work it out Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted July 20, 2023 Share Posted July 20, 2023 Is the dash valve giving an issue? Quote Link to comment Share on other sites More sharing options...
Will Posted July 20, 2023 Author Share Posted July 20, 2023 I replaced the dash valve 6 months ago, surely it’s not buggered. I can’t work out why when I push in the foot pedal the valve stops leaking? Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted July 20, 2023 Share Posted July 20, 2023 I'm not good about the air line plumbing diagram in my head. I would need a diagram to look at. But is it possible that a couple lines are crossed.. 1 Quote Link to comment Share on other sites More sharing options...
Will Posted July 20, 2023 Author Share Posted July 20, 2023 Nope, it’s been all fine until today Quote Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 In North America, there are two other places that the two systems interact. One is the inversion valve and the other the anti compounding. The 1st is a system that if you loose one side of the dual system, uses the spring brakes proportionally applied by the other half of the duel system. It vents spring brake air in relation to brake air on the other system. So there is one place that could leak spring brake air into service air piping. The anti compounding is there to prevent both air service brake and spring brake from being applied at the same time. (prevents too much pressure on the cam and slack adjuster). On both of these it could leak into the other system. If Your leak is a minor one that seams to go away when the service brake is tapped, than I'd look at those two valves If on the other hand, it leaks all the time the spring brakes are off and the service brakes are off, and only goes away when you step on the brake, Then it is likely a relay valve. 1 Quote Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 Also you only mention it happening when the "yellow button" is in, I assume this is a single unit (red button out) when it happens? If you have a trailer there is more that can leak with the red button in. Quote Link to comment Share on other sites More sharing options...
doubleclutchinweasel Posted July 20, 2023 Share Posted July 20, 2023 Could the parking brake release air be leaking through a brake can and coming out the service side? Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 If I understood, he checked for that. Quote Link to comment Share on other sites More sharing options...
doubleclutchinweasel Posted July 20, 2023 Share Posted July 20, 2023 Just now, Geoff Weeks said: If I understood, he checked for that. Oh. Yeah. Sorry. I missed that. Too early in the morning. Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 Also if this truck has anti-lock brakes there is another whole can of worms. Quote Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 31 minutes ago, Will said: I replaced the dash valve 6 months ago, surely it’s not buggered. I can’t work out why when I push in the foot pedal the valve stops leaking? The reason it stops, is because before you apply it is venting air that the valve doesn't think should be in that line. It is like when you had you foot on the brake, and let off, all the air doesn't go back up and vent out the foot valve, it would make the brakes very slow to release. so quick release valves or relay valves, when they loose service brake pressure from upstream, vent everything down stream. Once you supply service pressure from the foot valve, the vent is closed and pressure allowed to build in the service line to the chamber. How big is this leak? does it vent all the air and the compressor can't build until you step on the brake? or is it a small leak that stops when the service brake is applied? Quote Link to comment Share on other sites More sharing options...
Will Posted July 20, 2023 Author Share Posted July 20, 2023 It’s a big leak, if I don’t step on the foot pedal it’ll leak all air out of the system Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted July 20, 2023 Share Posted July 20, 2023 Is the brake valve a ''full function'' valve or something like an R-12 valve that is only providing air to release the park brakes? Quote Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 May be I am mis understanding here. My understanding is that it is leaking out back on the frame by the brakes, not at the buttons on the dash. A photo of the valve that is leaking would help. As previously stated, there a a few places the service and spring brake systems interact. knowing where the air is coming out will help. It may not be the valve that the air is coming out that is the problem, it may be another valve that is letting spring brake air into the service brake lines. Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted July 20, 2023 Share Posted July 20, 2023 frame mounted valve is leaking, and is new.. Pics always make it better, Im leaning toward an incorect plumbing issue, .. Glad your here Geoff.. Jojo Quote Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 20, 2023 Share Posted July 20, 2023 There is a line from the brake service line to the spring brake relay with anti compounding or separate anti compounding valve. If that valve is defective it will pipe air for the spring brakes to the service line to the chambers, the quick release portion of the service relay will be open and will dump the air. When the brakes are pressed the 1st thing that happens is the quick release portion of the service relay closes. Without being there and seeing exactly what is happening, that is my best guess with the info we have now. Inversion valve is another possibility. 1 1 Quote Link to comment Share on other sites More sharing options...
Swishy Posted July 22, 2023 Share Posted July 22, 2023 Not sure what's go n on but in some cases the rubber 'O' rings B Tween the 2 chambers start leaking in the maxibrake and when U release the parking brake by pushing the big button in on the dash air leaks into the service brake chamber and gently applies the brakes on the oposite wheel or leaks out the quik release valve other way of checking is take the service brake hose off the maxi and see/hear if air is leaking out the brake port on the maxi ThemsTheBrakes cya 2 Quote Link to comment Share on other sites More sharing options...
Geoff Weeks Posted July 22, 2023 Share Posted July 22, 2023 The O/P'er said he removed the line and checked for that. It is a good first thing to check. 1 1 Quote Link to comment Share on other sites More sharing options...
Popular Post Will Posted July 22, 2023 Author Popular Post Share Posted July 22, 2023 Problem fixed, ended up being a brake booster. Thanks for all your help! 1 1 1 Quote Link to comment Share on other sites More sharing options...
Popular Post 70mackMB Posted July 22, 2023 Popular Post Share Posted July 22, 2023 Will! Thank you for posting what the problem was. Too many never let us know "The rest of the story". .....Hippy 4 1 Quote Link to comment Share on other sites More sharing options...
fjh Posted July 23, 2023 Share Posted July 23, 2023 14 hours ago, 70mackMB said: Will! Thank you for posting what the problem was. Too many never let us know "The rest of the story". .....Hippy Agree totally with this statement ! spring brake is the most common failure with this symptom, there are a few other issues that can happen How ever yours fell in to the most common category ! Thanx for posting the fix! 1 Quote Link to comment Share on other sites More sharing options...
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