Since I have recently discussed the secondary pouch board, I will endeavor to avoid redundancy and keep it short for this post.
The main difference with a primary pouch board is that it will have a row of “bleeds”; brass cups which bleed a tiny amount of suction back into the area underneath the pouch, to ensure prompt resetting of the valve after the atmosphere signal is cut off.
The bleed cups may be in good condition, or they may be tarnished or even corroded.
If they are damaged or corroded, they should be replaced; if they are simply cosmetically tarnished, then the main thing is to ensure that the bleed hole is clear and functional, and that the seal around the cup is still good.
The bleeds are indicated by the green arrow in the following image:
The red arrow indicates that the last pouch is lifting away from the board, at its edge.
Check the pouches for damage or lifting such as this, and repair accordingly.
As with the secondary, check that the pouches are tight (covering each bleed in turn), and that the channels are all sealed independently.