My 'ex-s' 454 has had to have both rear calipers changed as sometimes
the handbrake worked, other times not...and she also said the car felt
as if handbrake was on even when it wasn't.
It turns out that the handbrake side of the calipers were sticking every
now and then....
BUT...it also transpired that the rear flexible brake
pipe hoses were braking down internally and collapsing, which was
also stopping the pressure being released from the caliper pistons !
Believe it or not, this is not an unknown or uncommon fault for most cars
that reach a certain age or point in their 'life'
Rob at S2 told me he has seen quite a few 454's with this particular
problem, and has replaced both rear hoses on about a dozen 454's
so far. So....if the handbrake mechanism appears to be ok and free
then it's 99% more or less a problem with the rear flexible brake hoses.
NOTE:- this doesn't appear to be a problem for the later/final batch
of 454's that had drum brakes rather than disc brake rears..
such as the 'Coolstyle' model. The Brabus was NEVER ever
released from the factory with drums...just in case anyone
wondered.