Weak rear brakes

I'm working on a problem with very weak rear brakes. I found a thread in the Gen 1 forum where the problem sounds very similar to mine. The OP replaced the LSPV to fix the problem. My LSPV piston can be pulled down, and springs back up easily. To me this indicates that it is not plugged, and obviously not seized.
Has anyone seen an LSPV failed, but the piston moves as it should?
I found a couple of other possibilities in my research. I'm going to check to make sure the master cylinder rod is retracting all the way, and I'm going to have someone step on the brake pedal while I watch the LSPV piston to see if it moves. If it does, I would think that it is functioning correctly.
The brake pedal feels good to me. It's been some time since I did brake work on the truck, I don't recall if I bled the LSPV. I considered the possibility that there could be air in the interconnect / return line, but I would think the pedal would feel spongy regardless of where air might be in the system. The LSPV spring arm was raised to the point that I should have had max rear brakes all of the time. I found out my rear brakes were not working well when I developed a leaking wheel seal, and pulled the brake drums. Virtually no wear on the shoes, they've been on there for years.
IIRC, last time I worked on the brakes, it would lock up all 4 tires on a dirt / gravel driveway.
Any help would be appreciated.