There is a product called Halt! which is carried by letter carriers a lot for dog problems. Its basically a pepper spray in a small canister with a belt clip and runs less than $10 at places like WalMart. That might be worth checking out. Just don't spray yourself or you'll end up in a vegetable garden! That stuff hurts!
I did a little research online regarding pepper spray for dogs. Halt huh? I know your Pop had a bad experience with ammonia but my Pop swore by it. Just the scent of ammonia seemed to back off that pit pull. Let's say I'm not overly confident of my ability to 'shoot' straight while trying to maintain an upright position on my ride. With the pepper spray I need to hit em in the eye for it to be effective don't I? Pepper spray's only good for about 10 total seconds right? What if I'm a really bad shot. With my bottle of ammonia I'm good for at least a hundred good shots. I might try pepper spray though. Do you suppose the dogs will learn after being sprayed a few times?