I've got a Shimano Tiagra front brake and a Bianchi Pista. When I screw on the brake it's got about 2.5cm of play. Is there some kind of washer/spacer to use to mount the brake?

yeah there is,when i replaced my front break there were some washers on it.i surgest asking at your local bike shop,im sure they can help you.

Or if you want to save money just get standard washers from hardware store in right diameter.

The allen nut comes in different lengths. The one for carbon orks is deeper than the steel one.

Should be available from any lbs, or here:

http://www.chainreactioncycles.com/Models.aspx?ModelID=17557

encountered this problem at work the other day. the pista forks run kinda shallow.
we fitted a rear brake with a slightly longer allen nut.
worked a treat

Do you know what length of nut would be needed to mount the front brake? I don't really see how a longer/shorter nut would work because the nut stops moving when the threads on the bolt get to the end of the threads on the nut so no matter how long it was it'll always stop at the same spot. Do I need to cut the bolt? I really don't want to have to do that.

Does this nut have a kind of plastic thread gripper at the end that is stopping it going further?
Is the bolt too long for the fork, so the nut runs out of thread before it touches the fork?

The reason the nut stops is because the allen bolt fitting is narrower than the thread. the bolt wont fit through the allen bolt hole so when the threads on the nut stop, the nut stops moving. The nut doesn't run out of thread. It's more the other way around.

It's the lowermost part of the pivot bolt that seems to be too long. You can see it here. It's the bolt with the two threads.

Would using the rear brake pivot bolt work?

Dunno man, I still can't quite picture what's going wrong. Fsck all sleep doesn't help clarity.
Can you take a pic of your bike? It sounds like you need a different bolt but I'm really not sure. Is the fork hole drilled large enough?

I'll get some photos tonight and post them up tomorrow morning

dont forget a rear brake has a much shorter thread than the front... (since a rear brake bridge is much shallower in depth an a contemporary fork. drilling.. )

we ran a rear brake with a longer allen bolt... as opposed to cutting the front brake thread shorter. we didnt really measure anything, just got a allen bolt to fit after trying a coupla different length ones -

i'd suggest returning your front brake to the store you purchased it from and exchange it for a rear, and get them to give you a longer allen nut to fit... hey thats whats your lbs is for...

hope that helps a little...