A lot will depend on the driver. There is a lot of steam in the circuit between the regulator valve and the cylinders, that's why on opening the regulator nothing happens for a few seconds. I stand to be corrected but from observations on Bulleid Pacifics the correct procedure is to open the regulator about halfway then almost shut it, wait for the first sign of movement and then gradually open it further. I don't doubt that any other Pacific will be the same. The slipping reputation of Bulleids probably originated with the unrebuilts when there was a lot of leakage from the oil bath splashing all over the place. Why did Bulleid insist on welding the sides of the oil bath to the frames that will flex and inevitably lead to leaking weld seams. At least with riveting there is a little bit of 'give'.