Just replace the bag. Unless you literally submerge the bag in water with air in it at pressure, you can't get the soapy water everywhere, especially up inside it. Unless they have been replaced in the last 5 years that you are sure of, just replace one or both. It is a very easy job and can be done in 30 min or less. Lots of online instruction. You will be chasing it forever, when in a short time you could replace it and be done.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600