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tractor conversion to dump

BLconst

Well-Known Member
Joined
Dec 31, 2008
Messages
68
Location
So Cal
I have a 97 pete 10 wheel dump that used to be a regular tractor. When i originally purchased it i noticed that when loaded i had very little braking power, but when loaded and towing a trailer I had normal braking power. I have a caps that I put on the air lines to the trailer gladhands(when not towing a trailer) and if i depress both tractor and trailer brake buttons the brakes are fine. I have ran like this for many years. I have learned that a regular tractor without trailer, the air to rear wheels is decresed to prevent lock ups, now that the truck is a dump, it needs full air to rears all the time. What is the name of the valve and where is it located to change to convert from "tractor" to "truck"? Im afraid someone else will drive the truck and not push the red button in and have little brakes.
 

Willis Bushogin

Senior Member
Joined
Nov 6, 2007
Messages
855
Location
NC
Occupation
owner
You know, I have owned lots of dumps trucks in the last 15 years and at present I own (4) I just bought a 89 Freightliner, converted Tri-axle dump truck. I have converted them, but never had that problem. The Freightliner I have now, has air trailer brakes (I dont have a trailer, so I dont use it) Stupid question, you dont have a leak on your glad hands do you? without the trailer hooked up.
I think the valve you are talking about is the proportioning valve (bad spelling) You should be able to trace the trailer air brake lines, back to this valve, they are all located in different places. Maybe this valve is leaking internal, bypassing, or maybe you are correct, but it doesnt sound like it should work like you say it is.
Think about the safety issue, I dont think a manufacture would make a system like this, because as you stated, the only time you have real good brake is with the trailer. I dont think this is normal, you need great brakes on the truck all the time. I always adjusted my trailer brakes, to match the truck brakes, to keep them from locking up.
Good Luck and I look forward to reading other comments
 

k.key

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Joined
Jun 30, 2009
Messages
54
Location
Indiana
Occupation
Mechanic Self Employed
BL, Your truck could have a bobtail brake limiting valve or a bobtail limiting relay valve on it. If you give me the last six of your VIN# I will check it out for you next week...
 

mitch504

Senior Member
Joined
Feb 27, 2010
Messages
5,776
Location
Andrews SC
A bobtail limiting valve is standard on modern Mack tractors, the Mack airbrake manual has an excellent diagram of the difference in tractors and trucks.
 

BLconst

Well-Known Member
Joined
Dec 31, 2008
Messages
68
Location
So Cal
vin 422456 i looked up on bendix web site and it does seem like i have a bobtail proportioning valve. It doesnt say anything about what to do when you no "bobtailing" which a dump isnt bobtailing its a truck with a load now.
 

k.key

Well-Known Member
Joined
Jun 30, 2009
Messages
54
Location
Indiana
Occupation
Mechanic Self Employed
BL, It sounds like it has a limiting relay valve, which would make it not want to stop without a trailer, and really not want to stop with a load in the bed. You can use that relay valve if you apply constant air pressure to the control port on top of the valve, or you can replace it with a standard relay valve. Heres what I would do, plum a tee into the supply port of the quick release valve on the parking brake side, run 3/8 plastic line to the control port on the relay valve, cap off the line that originally went to the control port, that line is why you have brakes with the red button pushed in. Doing this when you release the tractor brakes you will be applying air to the control port. There should be two ports on top of the relay valve 1 control and 1 service should be marked, follow your hoses from your piggyback chambers the service brake line should lead you to the relay valve, the parking brake hose should lead you to the quick release valve, the top port is supply on the relay valve. If you verify that it is a limiting relay valve using the Bendix site, you could just take it off take it to a parts store tell them you want a standard relay valve that will work in its place.Good Luck
 

k.key

Well-Known Member
Joined
Jun 30, 2009
Messages
54
Location
Indiana
Occupation
Mechanic Self Employed
BL, on second thought I would probably replace that valve with a standard relay valve, my concern is that relay valve has constant supply air to it, if it were to fail it could feed air to the quick release valve causing park brakes to release, unlikely but. We don't want that, call the Pete dealer get a part # on your relay valve, there are two crack pressures, depending on part #.You could run another line from the tanks to the control port on your valve , but I think I'd just get it off of there.
 
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