About two years ago my '90 240 sedan suffered from a stuck right rear brake caliper. I replaced both rears with rebuilt ones. Last month it suffered the same failure. I remember from reading on the forum here that this seems to be a common issue with rebuilt rear calipers so this time I thought I'd use a different rebuilder and keep my old ones to do a post mortem and perhaps rebuild them myself with quality components.

After installing the new ones all seemed fine for about a week when my son reported an instance where it stuck again. After stopping, the symptoms went away and it seemed fine again and hasn't stuck since.

This led me to speculate that the problem ma lie elsewhere. It made me wonder if it may be an issue with the brake proportioning valve. I have never heard of one failing so I don't know what the symptoms would be.

Any thoughts on this being the root cause of this recurring failure?