Shrek is right, brassica seeds are small and require very little cover. I believe suggest planting depth for most brassicas are under a 1/2in.
If your going to plan on working up this ground again, possible discing may help loosen soil and make your tilling a lot deeper and quicker. Ideal would be some kind of plow,, if you have the equipment, space, ect...
Waiting till next spring and really trying to get it tore up with a disc and tiller when the ground is wetter and softer may be a help aswell. Once you have the ground tore up GOOD one time it makes it a bunch easier to break up again. I have a new piece this year I had a guy till up. I figure next year I will be able to use an ATV disc a few times through out next summer to keep it dirt.