The rear rotors on the '91 Legacy exhibit some rust on the braking surfaces. The rust is chewing up the pads. I swear I didn't notice anything 6K ago when I rotated the tires. I know the fronts do most of the work (and look great, clean and shiney), but is it common for the rears to junk up or could there be a problem with hydralic pressure?

A metering valve seems to be about $300 so I don't want to go there yet. Is there an inexpensive brake pressure gauge out there somewhere?

 

Any other recommendations are welcomed too.

This is pretty common on disk brake vehicles. I've had Pathfinders and a Cressida do the same thing, especially on the rears. What it amounted to was lack of regular preventative maintenance. That is, every 3 years flush and replace the brake fluid on the car, bleeding each caliper inturn. That prevents rust seizing up the calipers. Twice a year, pull the pads (front and rear), remove the calipers (don't disconnect the brake lines, just hang the caliper up onto the coil spring with a hook made from a clothes hanger), and remove the caliper mounting brackets. Clean and wire brush all parts. Then lubricate all sliding pins using silicone grease (won't attack rubber boots on slider pins), put antiseize on all mounting bolts ( makes the job easier next time), put antisqueal compound on the back of the pads, and reassemble. This prevents the pads from sticking, or sticking on one side only, and ensures that your rotors get even pressure and wear. Done regularly you shouldn't see the rust pattern again, if the car is driven regularly. I'll bet you don't have an unbalance in your system at all.