I'm not sure if big red do it, but you can also use alloy pistons in them like I did with a set years ago, must admit it is on a car that is stood up for months at a time and not a stick in sight, 10 years on.
Big red do a ace job though, I've resealed lots over the years, a lot of the time it has been a waste of time without new pistons, I'm sure they mishape slightly with the years.
tabetha