Got a 3.5 month old trailer with very sensitive brakes. The trailer will leave skid marks with just over moderate pressure on the brakes mostly when empty. The truck tires are not locking up. The dealer has adjusted the brakes but that didn't fix it. Any idea where to look. The trailer is a 25 ton, triple axle, air brake, tag trailer. Thanks for your time.
you said it's a new trailer . it should have automatic slack adjuster's on the brakes so if the dealer did not adjust the automatic adjuster and only backed them off manually then they just re-adjusted in a couple of stop's back to where they was . that's the problem with them and empty trailers. does it do it when loaded ?and do they release immediately when you let off the pedal or does it have a delay before it releases the brakes
ABS sensor?
Couple possibilities. First, have you pulled a trailer with good brakes with this truck before? If not, you could have a bad tractor protection valve on the truck. It could be pushing too much air to the trailer. Get a gladhand to hook onto service line on truck and put a pressure gauge on it. You'll have to plug the emergency gladhand so you can release the trailer brake control. Have someone lightly step on pedal. With light to moderate pressure on pedal you should only have 10, 15 lbs maybe. If you're getting 20-30 lbs or more then could be problem with truck brake valves. Also the service brake valves on trailers can have different cracking pressures. Maybe the wrong valve was installed on trailer. Also something to check is are all your brakes on truck properly adjusted and functioning? If you have only half your brakes on truck you're going to be pressing harder on pedal to stop which will be sending too much pressure to trailer.
Is the OP still watching this thread?
Yes, I am still watching. When I hooked my other Dump truck it doesn't do it. So it appears to be truck related rather than trailer but I am interested in anybody's opinion of how to fix. Thanks
My first guess would be your tractor protection valve. Do you know if the trailer plumbing on the truck is factory or did someone add it later? Maybe they have the signal line from the tredle valve to the T.P. valve plumbed out of the wrong port?
Is the ABS working? When they 1st started putting ABS on the tiltdecks we had a lot of complaints about skidding the front 1 or 2 axles when they were empty. 90% of the time they were older dump trucks that didn't have a key on hot wire in the plug to turn on the ABS module. Just a thought.
Maybe I am missing something but I don't understand Shimmy's comment, and would like to explore it further. I don't see how the tractor protection valve can have anything to do with this, as there is no valve anywhere in an air brake truck that lowers the pressure of the service application except the bobtail proportioning valve, which is only found on a semi. Also there is the small amount of crack pressure in the relay valves which there should only be one in this path, on the trailer. Otherwise full treadle pressure (whatever your foot is doing) goes to the front brakes, rear brakes, and via the double check to combine front and rear application as well as the Johnny bar if it has one, to send it to the trailer. Trailer braking force is changed by brake drum size, camshaft profile, slack adjuster length, and air chamber size. This is all engineering by the trailer maker. ABS is another variable and I wonder if the different truck didn't have the trailer hot line available as suggested. That would be worth a look. However if it takes less psi from the brake pedal to make the other truck stop then that could also be the difference between locking the trailer brakes and not.
I believe all trucks have a tractor protection valve. This valve has a inlet from reservoir, signal line from pedal and johnny bar(if equipped), and both the lines back to the trailer come from this valve. From the other comments I believe its probably related to the ABS.
I used to see that with a truck with maladjusted brakes .
I heartily agree, that is the #1 item to check, otherwise the excessive brake pressure it takes to stop the truck will lock up the trailer, good comment.