how to open pr2 gripper with moveit_commander

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Hi, I cannot get move group to open gripper of pr2 eventhough I can successfully move the head torso and other parts of pr2 using the MoveGroupCommander. See the code below with two basic functions: move_head and open_gripper. This does not work when I try to open the gripper's hand. Note that the gripper's hand closes when it is opened and I set all the joint values to zero. But I can't get it to open when the gripper's hands are closed.

#!/usr/bin/python
# coding: utf-8
import sys

import moveit_commander
import rospy
import roscpp


def home_head():
    mgc = moveit_commander.MoveGroupCommander("head")
    jv = mgc.get_current_joint_values()
    jv[0] = 0.0
    jv[1] = 0.5
    mgc.set_joint_value_target(jv)
    p = mgc.plan()
    mgc.execute(p)    


def open_right_hand():
    mgc = moveit_commander.MoveGroupCommander("right_gripper")
    jv = mgc.get_current_joint_values()    
    jv = [1.0, 1.0, 0.477, 0.477, 0.477, 0.477, 0.04]
    mgc.set_joint_value_target(jv)
    p = mgc.plan()
    mgc.execute(p)

if __name__ == "__main__":

    node_name = "ready_pr2"

    moveit_commander.roscpp_initialize(sys.argv)
    rospy.init_node(node_name)
    rospy.loginfo(node_name + ": is initialized")

    rc = moveit_commander.RobotCommander()
    rospy.loginfo("robot commander is initialized")

    home_head()
    rospy.loginfo("head homed")  

    open_right_hand()
    rospy.loginfo("right_gripper opened")  

    moveit_commander.roscpp_shutdown()