From experience (b767,b747)number of issues with ata 36, never dismiss HP valve failure, try lubricating and exercising the vlv position switch arm(sometimes the problem is very obvious)
Controller and sense lines also prevalent(beware of unions that `weld`together) another favorite is the wiring ,chaffes are very common. The Earth line is also problematic ,ask your Tech services can you try a local earth.
The usual T/Shooting (in reality)goes along the lines of vlv,controller and then wiring.
Another good tip review cmc /ahm the snapshots can sometimes be enlightening.
As has already been discussed eng at low power settings the risk of overpx highly unlikely.