Pressure plates and levers do
not send continuous signals. They send an "on" signal on the step they get triggered and an "off" signal on the step they reset (when the trigger condition has ceased exactly 99 steps ago). They do absolutely nothing the rest of the time.
Bridges and other delayed-reaction buildings can get "wedged" in a state not corresponding to their trigger: when a new signal arrives during the waiting delay of an action, the new signal will be ignored and only the already-lined-up older signal will "take". This is a pretty common occurrence when trying to regulate fluid height with them, since single-height fluctuations are enough to jam the building either way and are hard to prevent.
Only doors and hatches, with their zero delay, are guaranteed to be synched to their trigger.
A simple way to generate fluid of a desired height would be to build a seven-tile area and place a mechanically-operated door in one of the tiles. Fill the area to capacity, shut and open the door once. One tile of 7-high fluid is destroyed by the closing door, when it opens again, the remaining six times 7/7 fluid spread over seven tiles, giving uniform 6/7 depth everywhere. You might have to drop the cart in from above to allow filling - when entering on track, it might remain too fast to pick up fluid.
I find it preferable to use "flat ramps", i.e. ramps that allow level change without providing acceleration, for magma-filling: use a one-tile wide magma trench engraved with track right through. Since both track branches lead to the level above, no acceleration/deceleration takes place, but the cart can still change level on them. I use a simple collision system - at least two magmasafe carts, which push each other out of the magma trench and then glide into the trench, by the simple expedient of a single impulse ramp pointing into the trench (on the level above, on the "entrance" side). Quite reliable:
http://mkv25.net/dfma/movie-2733-multi-cartmagmadelivery