Caliper seals.

Not unless you want to end up using the back of a truck to stop!

New pistons arrived, does it matter that they're "hollow" like this ? The ones I took out were solid..

Not solid but pluged.

Okay off the wall question coming: Why did they plug the piston rather than leave it hollow?

Rick H.

The plug is for heat insulation to prevent steam bubbles in the break fluid.

Outstanding, now I know. Always wondered about that item.
Rick H.

Thanks sirocco, you are a font of knowledge. I'm gonna use them, do you think they'll be a problem ? I've rebuilt the master cylinder, rebuilt the calipers with all new seals and those pistons, and fitted new steel braided lines. So basically all new !!