All 240sx brakes lock up

[snowyred180]

Ok, so I've had this problem plaguing my '91 240sx (non-abs, KA24DE) for the last year. I've searched. I've replaced most of my brake system. I'm pretty much at my wit's end on this one.

My problem is after about 10 minutes of driving, all 4 of my brakes lock up. I've replaced the front calipers with known working ones, the brake master cylinder (new), the brake booster (new), the soft lines to the calipers (new), and the check valve (used). Pretty much all sources point to the check valve, even though I've tried 2 I've snagged from the junk yard. I'm ordering a new one from Nissan now. When I pull the vac line from the booster, the brakes release, and I'm good for another few minutes.

If it's not the check valve (I've tried 2 already), what else could be causing no release on the vacuum?

When the brakes lock up, the pedal slowly gets stiffer and stiffer, until the brakes all drag. And yes, it is all 4 brakes.

I've replaced pretty much the entire brake system except for the hard lines and the prop valve. What else could it possibly be!?!?!