There are a number of problems that can exist when a “pressurization bump” is reported upon takeoff. At Duncan Aviation, we have worked on aircraft with as much as a 1600 to 1800 FPM cabin rate of descent.
When troubleshooting, don’t overlook the opening of the L/H main air valve.
On a 750, 800 (with mod 253201 incorporated), 800XP, 850XP and 900XP, the L/H main air valve is pulsed open by the Pressurization Air Control Box (ZT). If the opening of the L/H main air valve is erratic (usually a sticky valve) or not driven open in the proper sequence by a defect in the Control Box, significant “bumps” can occur.
While you are troubleshooting, take a few extra minutes to ensure the proper sequenced opening of the L/H main air valve. When performing the check, ensure the duty cycle of the valve (one minute of operation followed by a four minute rest period) is not exceeded.