Start simple, by removing the bolt in the lever and give it all a thorough clean, then grease it up.
If still no improvement, either the gold coloured bush could be worn in the lever (new lever needed), or the brakes could possibly need bleeding.
You can bleed the brakes on the quick by tying the lever firmly back overnight, with a cable tie or whatever you have, and any air bubbles will rise over night.
I tend to just loosen the reservoir cap when i do this but make sure you don't forget to tighten it back up before riding. I don't know if it's even necessary, it's just something i've always done?
Leave something on your seat to remind you though.
If that fails, then as Dazza says, it could be sticky pistons, or it could even be an expanding brake line if they're not braided?