leaking platypus valve

I'm wondering if there is a tear in your bite valve that is breaking the 'seal' and allowing water to flow?

Marcin, I own one too. EDIT: I should have read your post more carefully; It can not leak if the bite valve is above the waterline in the bladder, but it could if pressurized. Try placing your bladder lower in the pack if possible, and clip the bite valve higher on your shoulder strap.
A hydration system is very convenient, but after two leaking Plati I always carry some water in a PET bottle.

Brett, I'll try your suggestion of placing the valve higher, but I'm not sure it'll help – it's the pressure in the pack which makes the water come out. I can't see any visible tear or defect in the valve and was wondering if it's normal, or if should I get another valve.

The bite valve should prevent water from flowing out, regardless of the position of the bladder! I can hang a fully-loaded Platypus high up like an IV with the hydration tube / bite valve dangling directly below, and not a drop will flow down through the valve!

Water should only come out when you bite on the valve — or if you accidentally have something heavy (like your pack) pressing on it — both of which will open up the valve.

If you can't readily fix this, then you should return/exchange the valve or the whole arrangement if purchased together.

Do you have the 90 degree shut off valve between the hose and the bite valve? If not, they cost ~$7 and the bite valve will slide off of the tube. The other thing to try that helped me is when I finish drinking, I blow a good bit of the water back into the bladder, freeing the hydration hose and bite valve from fluid.

Ben's suggestion about the tear is good. The only time I've had a platy valve leak was when it developed a small tear that wasn't apparent until close inspection, or if the valve itself worked too far out of the plastic covering piece.