Ali's approach is the superior one...I've done the bleeder valve method for years without a problem, but lately, it's been deemed an inappropriate method by others more seasoned with brake repairs (potential for allowing air into the system), so I will forgo that method in the future.
The reservoir cap only has a pinhole to allow for air to escape, so requires a lot of force to push the caliper pistons with the cap on (trying to push all of that hydraulic pressure through that little hole). Removing the cap should ease the process quite a bit.
As Ali said, be careful with overflow...if your reervoir is already full, you can move enough fluid through the brake lines when pushing the pistons to overflow the reservoir.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car