When you come to bleed it the water will rise in the res when you take the top off or loosen the top bleed screw. As water levels out at the lowest point and might spill over the top of the res with being on the case floor.
Far better put the res/pump externally rear top will make bleeding very easy.
With the res top level with the case top, as you begin to fill the res you can partly fill the loop before you turn the pump on.
Keep adding water to the res about half way say till you can't add any more water. No turn the pump on you'll be able to add water as it bleeds.
When the bubbles stop turn the pump off fill res nearly to the top put the top on then turn the pump back on tilt case at different angles to get any trapped air out the loop.
Always put the pump on max rpm when bleeding gets the air out a lot better and less chance of trapped air.
Check for leaks as you bleed the loop.
Job done.