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What type of piping arrangement is this? main and branch or a continuous loop?
Well it's highly unlikely you have a branch and main system if your tubing is all 1/2" pex unless your main is the manifold. So I'm going to assume the problem radiators are on a continuous loop branch. If that's the case your description would seem to indicate that the loop is blocked down stream (return side) of the radiators. If I understand your description you isolated this loop and couldn't get any flow of air through it when you put an air compressor to it. The blockage could be due to a kink in the PEX. If it's concealed (buried in a slab?) your only option would be to repipe from the last section of radiation where you know you have flow (where you get water out when you bleed it).
Are you asking for a call? if so there is an additional charge. I'll post an additional service offer if you want to talk. Let me know.
Or just click on the call pop-up if one appears on your screen. I would call you so you so I need to know when you're available.
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