e9500v8 Posted July 13, 2010 Share Posted July 13, 2010 This is on a 1988 mack r688st I have replaced the foot valve ,tractor protection valve , the two-two way check valves at the foot valve .This thing is driving me nuts if I push brake pedal lights will stay on for a long time till air leaks off .I am an ASE certified diesel mech I have worked on macks for 15 years and this one is killin me . Aside of replacing all the valves on the truck I am at a loss any help would be great. Quote Link to comment Share on other sites More sharing options...
rhasler Posted July 13, 2010 Share Posted July 13, 2010 This is on a 1988 mack r688st I have replaced the foot valve ,tractor protection valve , the two-two way check valves at the foot valve .This thing is driving me nuts if I push brake pedal lights will stay on for a long time till air leaks off .I am an ASE certified diesel mech I have worked on macks for 15 years and this one is killin me . Aside of replacing all the valves on the truck I am at a loss any help would be great.Make sure the correct n/o or n/c switch is installed. Make sure the switch is good. If you have a trolley bar make sure it isn't pulled part of the way down or leaking air to the service brakes. Do you have a wiring diagram? Quote "Mebbe I'm too ugly and stupid to give up!" Link to comment Share on other sites More sharing options...
e9500v8 Posted July 13, 2010 Author Share Posted July 13, 2010 I know it is the right switch I have owned this truck since it was new and may have replaced it a few times but the problem started all the sudden. The trolly bar is not pulled down .If I release the air at the light switch they dont come back on so I know that air is not leaking over to the service side in a chamber or valve.. Quote Link to comment Share on other sites More sharing options...
e9500v8 Posted July 13, 2010 Author Share Posted July 13, 2010 I do have a wiring diagram but it is not a wiring problem it is in the air system for sure!! Quote Link to comment Share on other sites More sharing options...
rhasler Posted July 13, 2010 Share Posted July 13, 2010 I know it is the right switch I have owned this truck since it was new and may have replaced it a few times but the problem started all the sudden. The trolly bar is not pulled down .If I release the air at the light switch they dont come back on so I know that air is not leaking over to the service side in a chamber or valve..Sorry, I thought you were just working on it for someone else. Let me think about it and I'll see if I can come up with something. Quote "Mebbe I'm too ugly and stupid to give up!" Link to comment Share on other sites More sharing options...
rhasler Posted July 13, 2010 Share Posted July 13, 2010 Here's a piping diagram that may help but I'm not sure how accurate it will be. I circled the quick release valve in red, I wonder if it might be blocked keeping air in the circuit.R series without ABS with Full Trailer Connection.pdf Quote "Mebbe I'm too ugly and stupid to give up!" Link to comment Share on other sites More sharing options...
e9500v8 Posted July 13, 2010 Author Share Posted July 13, 2010 Thanks so much I will have to check that out tonight I know right where it is so it wont take long to check it out.Thank you Quote Link to comment Share on other sites More sharing options...
e9500v8 Posted July 15, 2010 Author Share Posted July 15, 2010 Well it is def not that quick release but I did do some more fooling around and when you have the trailer brakes realesed they dont stay on so that tells me that the tp valve is good but the air is releasing through the trailer service valve that is not right. What valve does it release through when you are bobtail the r12 valves and the foot valve right .I think I have a bad new foot valve maybe I am wrong I have been before Quote Link to comment Share on other sites More sharing options...
rhasler Posted July 15, 2010 Share Posted July 15, 2010 Well it is def not that quick release but I did do some more fooling around and when you have the trailer brakes realesed they dont stay on so that tells me that the tp valve is good but the air is releasing through the trailer service valve that is not right. What valve does it release through when you are bobtail the r12 valves and the foot valve right .I think I have a bad new foot valve maybe I am wrong I have been beforeIf you look at the diagram you can see the there is a line going from the "T" near that quick release valve to the spring brake control valve on the "C" port. If you disconnect that line at the spring brake valve and check to see which side the air is coming from (valve side or line side) it might help it narrow down. You should be able to do the same thing with the R12 valve. It wouldn't be the first time a new part was bad out of the box. Quote "Mebbe I'm too ugly and stupid to give up!" Link to comment Share on other sites More sharing options...
fastwolff Posted September 26, 2010 Share Posted September 26, 2010 This is on a 1988 mack r688st I have replaced the foot valve ,tractor protection valve , the two-two way check valves at the foot valve .This thing is driving me nuts if I push brake pedal lights will stay on for a long time till air leaks off .I am an ASE certified diesel mech I have worked on macks for 15 years and this one is killin me . Aside of replacing all the valves on the truck I am at a loss any help would be great.do you have an e3 bendix brake valve? Quote Link to comment Share on other sites More sharing options...
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