Your valve sounds like something inside is trashed. I have three of them and they basicly work well. I use a mini servo at 30 oz/in, and while they do need to push a little hard, it's within the servo's capacity. The servo does not struggle, bounce or rock, even at full pressure (120 psi). The servo will be pushed back when the radio is turned off if the brake was set to on. it should only dump air out the vent when the brakes are released.

I have found the best way to set up a Trim valve is with a gauge plumbed in place of the brake hub. The braking range is narrow and once past it, you can push in farther with no more effect. The gauge also helps to set the "arm" point as well. Once set up, disconnect from the gauge and plug in the brake hub.