[Tigers] Powertune 7 inch brake servo

[Mike Michels]

I just replaced a Lockheed 5-inch brake servo with the 7-inch "Powertune"
brand item which looks identical to Lockheed. Problem is that it seems to
stick, with an abrupt "catch" feeling on-and off-brake. Sometimes the brakes
drag slightly. I have confirmed that system is completely bled (no such
problem if vacuum host disconnected) .  Vacuum non-return valve working OK.
Rest of brake system in good shape. I see some discussion on other Brit car
sites about this, mostly regarding the air valve. Anybody?