Ian,
That system actually keeps residual pressure to the wheel cylinders at all times. When you shut the machine off you should be able to crack a bleeder and get pressure out.
I suggest you check the shoe adjustment. Lift the drive wheels, remove the wheel and put two of the lug nuts back on to keep the drum in place. Remove the 1/4" bolt and remove the access cover on the brake drum. At 4 and 8 o'clock set your brake shoe clearance at about.0020" as I recall. This is a **** system with the 2 star wheels. I like to go about 5 clicks left, then 5 clicks right until the shoe clearance is in spec..