Transit 2011 2.4 tdci RWD Backpressure.

[frund]

Top cover gasket rock hard and leaking oil. He removed injectors and top cover. I changed new gasket and 4 oil seals he put it back together, I had gone to Borth fishing. Had a look at it yesterday and noticed quite a bit of backpressure. I asked about new injector copper washers yes fitted as part of a kit. was seats spotless, he thought so. There is no blue smoke from exhaust, engine sounds good no unusual rattles. I listened this morning with oil cap removed, no hiss or spitting no oil spilling out and very little backpressure engine cold. He called in 3hours later after 40 mile trip and the backpressure was back
is Blow by the likely cause.
Any suggestions please