The filter was in fact after the condenser so the compressor pressure was not the problem. We opened up the filter and found disintegrated polyester fibre with no metal screen shield at either end: any failure of the medium would have caused the fibre to migrate. Another problem with this was the lack of either a blowout valve on the OEM compressor or a functioning high pressure cutoff switch, both design flaws in the car. The new compressor has a reusable blowoff valve for just these problems: if it releases, you merely clear the system and orifice tube and refill. The high pressure switch controls only the fan, not the compressor clutch.
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See the 700/900 FAQ at the drop-down menu above right.