I just had a very similar situation: full kettle, boiler making steam but wouldn't get past 10-12 psi (low pressure ajax), condensate return pump always kicking on and running. What was happening was no condensate was returning, forcing more make-up water to be added. Therefore, more water was accumulating in the vertical part of the main steam trunk that feeds the steam-jacket manifold. When the system was drained and cooled, I opened up the main line condensate float valve and found it had piece of scale stopping the float from operating the valve. Once all was reassembled, the boiler was once more fat, dumb, and happy!
Are you accumulating too much water and condensate in your main steam feed lines? That would prevent the bulk of the steam from reaching the jackets.
Glacier Brewing Company
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