Legendary Pokémon

>I make 5.000-10.000 steps on Pokewalker.
>I go to Poke Radar to catch some rare Pokemon that appears on bushes with 3 or 4 exclamation marks.
>Instead almost all the times only common Pokemon at bushes with 1-2 exclamation marks appear.

What the heck do I do wrong? That's why I have never caught any Flying or Surfing Pikachu.

You have to press the button at the right time in order to continue the chain and move to higher exclamation marks

And which is the right timing? How many seconds in each bush?