Only when cold sounds like maybe some moisture in the system, water will condense when it's cold and give you a "water bubble" that will get somewhat dispersed when it gets warmed up. Still somewhat spongy though. I'd do the turkey baster method and then bleed the system as step 1. If there is no change I'd replace the booster (I have two good used ones btw) and last do the MC. Unless you can find a used MC/Booster combo for cheap and do both.
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