Agilent LC1100 flush valve filter element

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Recently, one of the 1100 models in the laboratory was used for many days to analyze samples using buffer salt solutions as the mobile phase. After those analyses, the column pressure was quite high. The column was removed and cleaned for three days; today, when the cleaning valve was opened, it was found that the PTFE filter element was very dirty, so it was replaced. The pressure is now basically normal. The teacher said that this filter element can be treated with nitric acid and then used again; quite a few of these filters have been replaced for the two LC units. The senior sister checked online and said that it’s sufficient to use 20% nitric acid for ultrasonic cleaning for 15 minutes, followed by ultrasonic cleaning with clean water for 10 minutes. But I spent a long time dealing with it this afternoon, and those filters were still very dirty. I would like to ask the more experienced folks, how should I handle this?