Odd 4HP18 Fault

[Citroënbender]

This is the transverse format of the 4HP18, V6 driven.  It's in a car I like to drive and rely on to get the power down predictably. 

Kickdown appears correctly set, throttle linkage probably is correct, but the tech docs I've got don't say which position you set the trans in for getting this 100% right. Fluid level is spot on, but it has been dumping quite a bit.  Converter neck seal is not leaking, and filter was recently flushed/inspected; no massive contamination just a light film of wear "mud" and a very few coarse sparkles likely from parking pawl. Governor seals and filter cover o-ring renewed last week using genuine parts, without effect.

Symptoms:
• When cold, very slow initial 1-2 upshift under light throttle.
• When pushed harder then backed off, delayed 2-3 shift with period of no drive for short time.
• Intermittent episodes of flaring on light throttle 3-2 downshift

There is quite a lot of fresh weeping/wetness around the 2-4 servo cover.

What I'm thinking, is the 2-4 servo is not pressurising correctly.  Either pressure is leaking past the piston seal or its passage into/out of the apply/release chambers is being restricted.  These issues aside, it seems to drive quite nicely as always. 

Has anyone here, seen such a problem before? If so, was the root cause in the band piston area, or the valve body?  It's something of a mad week and I'm keen to get parts ordered up to fix through the silly season, without overspending or mis-guessing.