wacatak Posted December 22, 2013 Share Posted December 22, 2013 Ive got a 1991 rd 688 dump truck the brakes on the truck are good and act fine but when i charge the trailer brakes it leaks more air than the truck can make out what looks like a diaphragm on the brake valve. Any ideas on this? Can I take the valve apart to see what the problem is? Thanks Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/ Share on other sites More sharing options...
Mack Doctor Jr. Posted December 22, 2013 Share Posted December 22, 2013 U can take some of them apart. Most of the time they are full of junk and corrosion. Leaning it out may help for a while. Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-222552 Share on other sites More sharing options...
wacatak Posted December 23, 2013 Author Share Posted December 23, 2013 ok thanks i will give it a try. Thanks again Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-222726 Share on other sites More sharing options...
wacatak Posted December 25, 2013 Author Share Posted December 25, 2013 To start hope everyone is having a good Christmas. I have pulled the foot valve apart it was alittle dirty cleaned it all out and put it back together. I charged the truck brakes then the trailer brakes with no trailer hooked up and there was no leak out the exhaust on the foot valve so far so good. I then roll it out of the shop hook it to the trailer and the same problem as before it leaks out the exhaust valve and wont release the trailer brakes. Does anyone have a contact number for someone that has some good knowledge on the air system, Im just trying to wing it but I dont want to start buying parts with out an idea where to start. Thanks again Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-223099 Share on other sites More sharing options...
pmolitor Posted December 27, 2013 Share Posted December 27, 2013 Did you check brake chambers on trailer? Sounds like one might be leaking. If you've never worked on a brake chamber ask for professional help. There's a spring inside that can kill you. Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-223257 Share on other sites More sharing options...
gearhead204 Posted December 27, 2013 Share Posted December 27, 2013 try just hooking up just the emergency or supply air line leave the application or service line off then charge the trailer, then check for air coming out of the trailer service port. if air is present go to trailer axle(s) with dual chamber brakes and one by one remove the application hose on each can and check for air coming from can service port if you find a bad one let us know and we'll get you to the next step Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-223259 Share on other sites More sharing options...
wacatak Posted December 27, 2013 Author Share Posted December 27, 2013 ok thanks im going to hook it to a friend of mines trailer that i know there is no problems and see if the truck works with it. ill let you all know how it goes. thanks again for the help Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-223260 Share on other sites More sharing options...
wacatak Posted December 29, 2013 Author Share Posted December 29, 2013 well hooked it to another trailer and the truck is fine so now time to disect the trailer. Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-223592 Share on other sites More sharing options...
gearhead204 Posted December 29, 2013 Share Posted December 29, 2013 pretty sure you will find a bad spring brake chamber Quote Link to comment https://www.bigmacktrucks.com/topic/33973-1991-rd-688-brake-valve/#findComment-223596 Share on other sites More sharing options...
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