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Brake Lights Wont Go Out


e9500v8
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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.

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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?

"Mebbe I'm too ugly and stupid to give up!"

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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..

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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.

"Mebbe I'm too ugly and stupid to give up!"

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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

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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

If 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.

"Mebbe I'm too ugly and stupid to give up!"

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  • 2 months later...

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?

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