How to stop the steak from leaking juice

Pretty much...yes, but you can fix it!.

When you properly sous vide or very slow cook anything, you'll retain more of the myoglobin color because of the even cooking that often doesn't go above 140 at all. So a properly cooked steak like this will retain much more of its red colored myoglobin. Simply put, the meat will have more red juices to release! (Its a great, great thing about sous vide.)

As @Ronald mentions, the other thing is the resting of the meat. It's an important step as the muscle fibers relax after the heat is off and hold juices better then. An often misstep for the home cook is they allow a hot piece of meat to rest on a flat, solid surface. This causes the bottom of the meat to steam against the board, open the fibers in the meat more, and release the juices on to the board. Rest your meat on a raised baking rack so that it has air circulation all around it.

After a short rest - for most steak 10 minutes is fine, then you can cut into the steak. Use a very sharp knife to slice. The meat here is essentially a sponge and you don't want to compress it and squeeze out the juices. A dull knife will do this and you'll lose more juice on the cutting board again. Use a sharp knife and apply steady, even, but light pressure while slicing - let the edge do the work (if it won't, sharpen the knife more).

There's a few things that could be going on here:

• cut: not all steak cooks at the same rate. Really tender cuts cook faster than some of the tougher, more flavorful cuts, increasing cooking time by up to 50%. Cuts like tenderloin, filet, and loin (US) - Sirloin (UK) are more tender and cook faster. Denser cuts like sirloin, top sirloin, and bottom sirloin (US) and rump (UK) cook slower. If you are cooking a denser cut then you simply need to cook it longer.
• Pan heat: Most chefs have really good stoves that produce load of heat, most mere mortals have average stoves which aren't as powerful. Medium heat on a professional, or very high quality stove is hotter than the medium heat on an average stove, so try cranking it up to full blast. Also, make sure your pan is fully up to temp. I cook my steaks on a cast iron skillet, and I let it heat up for 10 minutes before I start frying steak
• Heat contact: even though steak is full of fat it takes some time for this to start working, so coating your steak with a bit of vegetable oil (not olive oil, it burns at high temperatures) will make sure it gets good heat contact

So my advice would be to get the pan hotter and cook it longer. Try adding one more minute per side.

EDIT: @kenny says that he is cooking loin, so assuming it's 3/4", or 2cm thick and looking for medium done-ness I would cook the first side for 5 minutes and then the second side 3 minutes. The uneven times are to make sure it cooks evenly. So you need a total of 8 minutes cooking time. I don't do the flip every minute method because the uneven times work for me, and it lets me do other prep.