Trailer brake and ABS

[Hammerhead]

(01-17-2018 )Starlight Wrote:  

(01-17-2018 )snailexpress Wrote:  

(01-17-2018 )Starlight Wrote:  Ah ha, very timely. I have a peterbilt at my shop right now with that axact same problem. I found the drive axle relay valve defective causing its crack pressure to be about 40 psi on the gauge before the brakes applied and then they applied hard. Crack pressure on this paticular application is supposed to be 5.5 psi. Have a look at that on your trailer. The abs valve which acts as the trailer relay valve as well might be your culprit. But confirm first that your control line has no leak as that could cause an issue like this too.

Could you please give me some info where to look for this relay, where it located usually.

Its your main abs valve on the trailer. Follow the abs sensor wires from the wheels right to the valve. Its usually mounted directly on one of the air tanks. But like i said make sure there are no leaks first

What he is saying snailexpress is to apply about 15psi of braking pressure on the trailer spike (trolley valve) and then verify that the pushrods are starting to extend out of the brake pots and applying the brakes. Most trailer valves have about a 7psi or so cracking pressure, and if the valve is all gunked up inside and not cracking until say 40 psi, then the pushrods will not extend very much, if at all at 15psi.
If this is the case, then because the valve is all gunked up, and not cracking until say 40psi as an example, then the brakes are apllying very hard almost instantly, then the ABS sees the wheels locking up, and releases the brakes therefore making you feel that the brakes are "falling off" when you are in fact applying them.