Warren,
The solenoids are the problem. There is nothing else that is causing your problem. Have you made up the wiring harness to exercise the solenoids? This is the only way that you will have complete success getting the system bled. You have a solenoid, probably only one, that is seeping and causing you your problem. Remember that the system has a normal operating pressure of 4800 psi and it has to drop to 2800 psi before the pump will activate. DanO made up a DIY that shows how to make up the wiring harness to excercise the solenoids numerous times.
Let me know if you want me to send my tools up to you to do your flush.
Brad