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Old 02-28-2011, 01:55 PM   #1
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Location: Willamina Or
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Question air brake conversion?

Has anyone converted the air brake system for a three axle "tractor" to a three or two axle "truck"? My Peterbilt has a warning sticker in the door jam that says (This vehicle is a tractor. It has a federal motor vehicle safety standard 121 air brake system arranged for a tractor operation only. If this vehicle is rebuilt into a truck or used as a truck the air brake system must be revised to permit it safe operation as a truck. Contact your Peterbilt dealer for instructions.) I called Peterbilt and was told that I need to have the bobtail proportioning valve removed and install a R 14 relay valve. My truck was a three axle and has been changed to a two axle. The spring brakes were moved to the rear axle and I have an R 8 relay valve that controls both the rear service brakes and spring brakes. I would like to do the work myself but I can't find the bobtail valve. Any help will help. Thanks Kenn
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Old 02-28-2011, 04:14 PM   #2
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Not peterbuilt, but on mine it was on top of the relay valve. The bobtail proportioning valve is activated by the red knob on the dash. pushing the knob in deactivates the bobtail valve. Trucks built after 95 have anti lock brakes so some vehicles got away from reducing the pressure to the rear brakes when bobtailing. If you do have antilock removing one set of wheel sensors will turn on the fault light
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Old 02-28-2011, 04:35 PM   #3
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kenn give me the last 6 of your vin # and i will check with pete to find out which valves need changed. are you going to pull a trailer with air brakes?
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Old 02-28-2011, 05:35 PM   #4
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Thanks Volvoman, the last 6 are 153648 and it is a 1982 359. I will be towing a 10,000 lb trailer with electric brakes.
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