They do make a test kit where you can watch pressure it would probably be cheaper to change it. Did you observe any difference when you bleed the system on the front calipers any pressure difference.
I have noticed the vacuum boosters if you ever changed it before your problem started. The replacements have different length rods which are adjustible use a straight edge and measure to the tip of the old one and set the replacement the same. You may have to set a straight across the hole.
You didn't mention doing this but may have done it when the problem just started.
Crack the bleed line on the side dragging does it have pressure a good indication.