HS-25B CREW LOST CTL OF THE CABIN PRESSURIZATION CTL IN ZOB CLASS A AIRSPACE.

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AFTER TKOF FROM YIP AND ASSIGNED A BEST RATE CLB; THE ACFT WAS DISPLAYING SOME ERRATIC PRESSURIZATION RATE INDICATIONS. WE WERE WORKING AT MAINTAINING PROPER PRESSURIZATION RATE THROUGH THE CLB AND REALIZED THAT THE AIRPLANE WAS NOT GOING TO PRESSURIZE NORMALLY. USING MANUAL PRESSURIZATION CTL; BACKED UP WITH OUR CHKLIST FOR AUTOMATIC PRESSURE CTL FAILURE; WE WERE ABLE TO MAINTAIN SOME RATE CTL. WE WERE ASSIGNED A HIGHER ALT AND A CREW DECISION WAS MADE TO DISCONTINUE THE CLB; TERMINATE THE FLT AND RETURN HOME. WE STOPPED THE CLB BTWN FL290 AND FL300. A REQUEST WAS MADE FOR A LOWER ALT AND TO RETURN TO OUR POINT OF DEP; WHICH WAS GRANTED BY ATC. WE WERE ASSIGNED FL280 AND HDG 270 DEGS A R TURN OF ABOUT 30 DEGS. A DSCNT WAS STARTED IN AN EFFORT TO REGAIN ANY CTL OVER THE PRESSURIZATION AND BRING THE CABIN TO A SAFE ALT; ANTICIPATING NO DELAYS IN DSCNT. ATC INQUIRED ABOUT OUR DSCNT REQUEST AND FLT PLAN CHANGE AND WE ADVISED THEM OF OUR PRESSURIZATION PROB AND REQUESTED EVEN LOWER. IN THE CONTINUED EFFORT TO REGAIN PRESSURIZATION CTL; WE DSNDED BELOW OUR ASSIGNED ALT OF FL280. WE WERE ASSIGNED FL240; BUT THE PRESSURIZATION PROB WOULD NOT ALLOW US TO HOLD FL240 AND THE DSCNT WAS CONTINUED; ALONG WITH AN ADDITIONAL REQUEST FOR A LOWER ALT. ATC GRANTED OUR REQUEST FOR LOWER AND WE WERE ASSIGNED 15000 FT AND A NEW ATC FREQ. UPON THE RADIO CHANGE; WE WERE ASSIGNED 11000 FT AND LEVELED AT THAT ALT WITH THE NEW CTLR. IT WAS AT THIS ALT THE PRESSURIZATION RATE WAS NO LONGER A PROB. WE WERE SWITCHED TO BATTLE CREEK APCH; WHICH GRANTED US A HOLD AT 7000 FT TO ASSESS OUR SIT AND REDUCE OUR FUEL LOAD. WE HELD FOR APPROX 45 MINS THEN RETURNED TO OUR POINT OF DEP AND LANDED WITHOUT INCIDENT. AFTER REVIEW OF THE SIT BY THE CREW; A STOP OF THE CLB SOONER WOULD HAVE LESSENED THE URGENCY FOR DSCNT AND REDUCED THE EFFECTS OF ANY PRESSURIZATION PROB.