[ROVERNET - UK] Brakes dual system

[geffandjulie]

After much component rebuilding, my brakes functioned safely. Then I fried 
the master cyl by leaving the engine running for 3h. After a new mcyl, 
minimally ok brakes again. Then I knocked off the front bits of the air 
control valve. Fortunately I had a spare and installed it. Brakes did not 
really work much at all: binding on, very still pedal, and very slow to 
release. Plus, even when working I was slowly losing fluid from the left 
reservoir. Now...my mechanic thinks the booster is duff.  Is this a 
reasonable diagnosis?  Any help appreciated.
Geff and Julie McCarthy
AvMedSafe
503-241-8468