I have had the locked brake caliper in the past on my 850's but today I found a new one (for me anyway.) My son's 94 850 rear brakes are not connecting with the calipers. Both calipers are showing surface rust. Last Sept I had new Volvo pads put on the car but no changes since then and no issues at time of the pad change.

I did some web surfing and see it could be an issue with the slide pins. I also see people hit the caliper with penetrating oil to loosen them up. But my question is since the calipers can lock up on the rotor and ruining them if driven can a caliper be frozen open still be safe. RIght now I cannot tell there is a problem based on the brakes only working up front. I am trying to avoid the cost of two rebuilt calipers if possible but will not do anything considered risky on my son's car. Is it possible for the issue to be some sort non-caliper part. Like I said, both rear brakes are doing this which seems odd to me.

Thanks for any suggestions.
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1996 855 TL Platinum Ed., 1995 854 GLT 5 sp, 1995 854 (RIP), 2005 V70R (badly missed)