Well, if you replace them, at least you can take that questionable part out of the equation.
They may, and are most probably good, but if your flow is shy at the rear??? put new ones, then you know for certain that they got full flow and you won't have to touch em for a while. Maybe you can take one off at a time and check them, but just as easy to put a new one back on,can't be that expensive, plus, then they're new!
I only had one on my truck, cost about twelve bucks.
You can test by have the rear up on stands, then have a helper apply the brakes normal/hard for a bit, release the pedal, and your rear wheels should be instantly 'free'. If they still got some resistance still, then the flex lines could be partially blocked.
Hope this helps