Have you tried contacting Dukes?

Dukes Intl
9060 Winnetka Ave
Northridge, CA  91324
818-998-9811

I know of two pressurization controllers that have been rebuilt by Dukes.

BTW, my 360 developed the exact problem as what you described.  I, too, tracked it down to the Dukes boost pump.  When the boost pump was on the problem went away – when off the pump was allowing air to get sucked in.  I replaced the pump and the problem went away.

Hal Woodruff
IV-P 119 hrs.