Ford Transit 2.5 diesel smokes and bangs badly after braking?

1996 2.5 diesel Transit (170k miles on the clock). Starting with the engine cold, if I brake to a standstill in the first 2 or 3 miles of driving, immediately after braking the van starts smoking very badly (black / blue - definitely oil) and banging loudly from the engine. The engine tries to stall, but with a bit of gas it sorts itself out after a few seconds. This will generally only happen once per trip, but smokes so badly as to blind anything behind me. I'm also concerned about the banging since it sounds pretty violent. Once up to temperature, the engine runs fine and doesn't cause any problems at all. The engine is using oil at slightly higher than normal rate. Once the oil level starts to get a little low, the fault doesn't happen so often unless I brake hard. I'm certain the problem is oil getting into one of the cylinders, but how? My best guess is that one of the valve stem seals has failed, but any other thoughts would be appreciated before I take the head off.