Burning the tanker's fuel is/was done on the tanker's booster pumps only. Consequently, the pressure surges, although frequent, were always well below 50 psi. I recall pressure fluctuations (which is a more accurate description) ranging between 15 and 25 psi as we flew along together. It wasn't done for fun. Range and/or endurance was enhanced significantly. The Herc guys did the same on the S Atlantic airbridge until the tanker had to head north.
And I would add that, provided the current proceures are adhered to, the Nimrod will not now vent while remaining in contact and "burning the tanker's fuel". After all, we would be burning our own fuel from a much lower level in the tanks and replacing that fuel to that same low level (which is well below the vent pipes and blow-off valves).
Regards
Ed Set