I agree with CodeMonkey. The only break that consistently works for me against guys with real strong legs is what he described. I'll try my own description of it in case you are having trouble visualizing what he is talking about.
1. Gi under the armpits or control one of his sleeves so you can stand without immediately being swept.
2. Once you are standing, be very careful not to get swept by grabbing both his lapels and kind of sitting a little and pulling him towards you a little. He'll probably try to grab your ankles and possibly put his knees in your chest to try and sweep you.
Now, you have 2 choices:
3a: If he goes for the sweep, it involves him opening his guard. If your quick you can walk in a bit, sit on him and prevent him from closing back up. Then there are a few ways to pass from there.
3b: Often they can try to sweep and reclose their guard after failure without you stopping them. So, once they give up on the sweep (timing is everything), put your knee right in the middle of their butt, grip their belt or pants at the waist and sit down into combat base. Keep your back straight, putting pressure on there crossed feet and keep pressure on their hips by pushing down and little away to really create space for your knee. But watch you balance! You'll probably get swept a bunch before you get it, but practice will take care of that.
4: Once in combat base his guard should be open. If not, it will be real weak and you can even pass without it opening there. I suggest learning the Margarida Pass to go in combination with this opening. It's a sure fire way to keep your opponent uncomfortable throughout the entire process