Hi Geoff
Where are you placing your springs? I have always found them to be very sensitive and very equipment dependent (one size or type does not fit all). I've also found them to be material dependent depending on the types of materials being used in the components themselves.
Also with the air tubes you didn't mention that the component needs to be properly weight distributed. The bladder products perform very inconsistent because of the distribution of weight most heavier components have.
When you do your testing do you use more than one bladder. Reason I ask is because using one bladder doesn't seem to work very well because there is less pressure being applied to one area of the skin than another. One part of the bladder may have 8 pounds sitting on it and another may only have a few ounces.
MG