This isn't an F-body question (working on an 84 Fiero), but I'm hoping someone here will be some help. The Fiero had been sitting up for about 3 years, and the front calipers were seized up. I replaced the two front calipers (the rears were ok), replaced the pads and rubber lines, and started bleeding the brake system. I have bled plenty of systems, and done lots of brake work before, but have never run into a problem like this:

The rear calipers bleed out fine (good pressure), but there is just a trickle coming out of the front bleed screws on both sides. Is it likely the prop valve, or the master cylinder? There was a lot of moisture in the system, so I'm thinking it caused something to seize. I have never had a prop valve issue on any car before (including many much older than this one). I did disassemble the valve (and a spare I had for comparison), but couldn't really tell much from that. With the endcap removed, you could see a small valve (it would barely move in the spare valve, not at all in the original one, but I'm not sure if/how much its supposed to move anyway.
I did have someone mash the brakes while the prop valve was disconnected, and got a good stream of fluid out both MC fittings, so I don't think its that. Any ideas????