[Healeys] Brake booster won't release

[2x2doc at gmail.com]

I'm redoing the brakes on my 1959 BT7. I've replaced the brake lines and  
added a Powertune brake booster. I bled the brakes using an Ezeebleed  
system. The problem is that the booster won't release once I've applied the  
brakes (it eventually releases but can take a couple of minutes) I've  
mounted the booster just like the instructions say. The brakes work fine if  
I disconnect the vacuum line (and plug the manifold). I've bled the brakes  
numerous times and it always hangs up. Anyone have any ideas?
Pat Williams