I have a tandem axle trailer with surge brakes and a backup lockout solenoid. The problem is that when my boat is on the trailer, backing it up is not a problem. When I have the boat in the water and want to back the trailer on flat ground or a slight incline the surge brakes lock up. Once I'm on the ramp and the trailer is downhill, the surge brakes release and all is fine. I suspect that it might simply be low fluid since I'm getting a "knock" when I'm braking in a forward motion and then pull away again. I will check the master cylinder, but other than that, what could it be?

Any ideas would be helpful.

Thanks.

Sounds like a wiring problem? Lockout solenoid needs to be wired to the Backup/Reverse lights, not the Brake lights.

If you're by yourself, put the tow vehicle in reverse and set the parking brake to make sure it doesn't go anywhere. As you connect/disconnect the solenoid wire (either the whole harness if it's wired that way, mine is a seperate wire right now, and the trailer needs to be grounded to tow vehicle), you should hear it clicking. You'll need somebody else's help for this, but if you hear the clicking when somebody hits the brakes, it's wired wrong.

I have had that problem show up once. I had surge brakes and the trailer sounded like it was hitting a submerged boulder or something when it was fully wet. I don't drive but 2 miles to storage so I didn't really care - I just wanted my boat in the water. It only happened once. I replaced the axle with the brake (it was bent anyways) and never heard it again.

It possibly being a wiring problem makes perfect sense.

Either your backup lockout solenoid isn't wired correctly as Michael stated, or you have a manual lockout where you have to physically put a little lockout key into a recessed channel each time you intend to backup to a point where the surge brakes would normally compress.

as other said , wiring , the click from the solinoid is not very loud , i'd have someone put a screwdriver or something in the general area where you think it is and put to a ear to hear for the click..

what kind of trailer ???

backup lockout solenoid go bad quite often, what is supposed to engage the solenoid? reverse lights?

Yea, the solenoid is wired to the reverse lights. It only blocks the brake fluid path, I have noticed if I come to a quick stop and then put in reverse the brakes can drag slightly until I pull forward slightly.

You can replace the blocking solenoid with a "bleeding" solenoid. Basically instead of blocking the line when you put the truck in reverse, you have a solenoid that allows ANY brake line pressure to be returned to the master cylinder via a bypass tube. (you will have to drill and tap a fitting into the back upper portion of your master cylinder) I have used these on several trailers and don't understand why manufacturers don't use them. Regular solenoids are about $30, these are only about $50.

As t500hps said.
Nothing is wrong with the solenoid or your brakes. It's just a bad design with the standard reverse solenoid. Its REALLY bad if your trailer has disc brakes. Drum brakes usually can be overpowered in reverse.
If you stop at the bottom of a hill downward, the trailer brakes are applied until you pull forward. Same as if you just stop at a light on level ground. By putting your truck in reverse, you are simply holding that braking pressure in the lines. Hence the no go backwards. The deluxe solenoid bleeds off that residual pressure as soon as it gets the signal from the reverse lights. It's only a few bucks more.

I disconnected the solonoid wire from the backup lights and wired it to a switch in the cab of my truck so I can engage it when I need it. I had a similiar problem trying to get it into reverse before the brakes would engage. PIA! Switch in cab is way to go, just engage the solonoid while brakes are off and they stay off while backing up an incline.