Another little Gem.
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If park brake was selected on and aircraft moved forward with 4 or more wheels rotating, pressing the toe brakes would have no effect as normal brakes are not avail in said condition.
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Parking brake or Anti-Skid to off, then toe brakes should work we would like to think.
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PS. Reason the centre gear don't work with parking brake is the centre gear leg is a different rake angle to mains, as aircraft gets loaded/unloaded all gear would be stressed a lot.