Basically, what you got is a piece of plumbing fraud. Nowhere in the United States is such a device usable as backflow prevention if there are any other valves downstream of it. All it is good for, is manual on-off flow, with atmospheric vacuum breaker protection, if it is at least half a foot higher than any heads or pipe it feeds.
Don't feel too singled out, you have company. Lots of systems were incorrectly plumbed, and done so to avoid the expense of correct plumbing. The correct device for backflow prevention, when it can be higher than the plumbing it feeds, is the Pressure Vacuum Breaker.
This still doesn't fix a leaking valve, and the quickest remedy is to buy a new valve and replace the bad one with it.