Surge brake lock out

[scottrus71]

Question about the surge brake lockout on my MFI 2009 trailer. When in reverse I understand the solenoid will prevent the trailer breaks from engaging.

But I can see the inner slide will still compress when backing up. I’m guessing that is normal for the slide to compress even though the solenoid is engage in reverse?

Is that right? The solenoid only prevents pressure from traveling down the line?

 

[Scottintexas]

Do you have the lockout pin in place or reverse light hooked up when you see the slide compress

 

[scottrus71]

I do not have the lockout pin. I do have the reverse light wire hooked up to the solenoid.

With the trailer wheel off the ground, I can manually actuate the cylinder without the truck connected and verify the brakes engage. That tells me the brakes work.

I guess my question is about how the reverse light and solenoid prevent the brakes from activating. It doesn’t appear to be mechanically since the slide compresses while in reverse.

 

[Scottintexas]

The reverse flow solenoid has a third port that redirects the fluid to the master cylinder that would be going to the trailer brakes. When the reverse lights on the tow vehicle activate the solenoid, the valve inside the solenoid redirects the brake fluid to return to the master cylinder.

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Copied from google search

 

[scottrus71]

Hahaha. Makes sense now why I see the slide compress while reversing but don’t feel the trailer brakes engage.

Thanks for finding that!

 

[ncnmra]

To confirm, this means that even if the brakes are engaged (ie: you had come to a complete stop and the trailer was compressed), as soon as you even pass the "reverse" position on the transmission, the solenoid will release the brake line pressure and the brakes will disengage, even if the actuator is fully compressed, right?

This also means that the brakes will not re-engage until the solenoid is deactivated with the actuator fully released.

I ask this because it would mean that if you park on a downhill, then the brakes will release because you would have activated the solenoid as you put the car into park (even if briefly passing through "reverse"). In this case the brakes won't come on until the tongue is fully extended again while driving.

 

[Seadeals]

I think your logic is sound but I don’t know that passing through reverse to park will energize it enough of long enough to bleed all the pressure off. I do think if you shifted into reverse for a few seconds then park, you are likely correct. I always carry chocks with me.