Goofy brake action on my Chevy pickup

[Rich69shortfleet]

My 69 C10 has 71 stock front disc brakes. Last Summer I had some serious brake issues and ended up replacing the rear shoes, the rear cylinders, the master cylinder and the front pads. I also had a lot of crud built up in the rear lines on the axle so I removed them and cleaned them out. I also cleaned out the proportioning valve as there was some small amount of rust in the fluid coming out of the old master. About the only thing I did not replace were the front calipers as they seemed in good condition. The proportioning valve is not available as a reproduction or normal part replacement. By the time I got done bleeding the system, it most certainly has fresh fluid throughout.

Just recently the brake warning light on the dash has been coming on occasionally and when it is on the brakes feel soft and don't stop as firmly as they should. If I press the pedal hard 2-3 times the light goes out and normal braking is restored. For a while. The booster is good, this is not an issue of a lack of power or a hard pedal. When pumped up the pedal remains firm and does not sink to the floor under constant firm foot pressure. I did 2-3 three hard stops in reverse to adjust the rear drums but this didn't help.

There are no fluid leaks at the cylinders or calipers or at the valve or master. The fluid levels in the master are constant.

Could this issue be related to the front calipers, the only non-new part in the system besides the prop valve (which I can't replace anyway)? Could a sticky caliper cause this? I just don't know what else to try. Maybe the master, new or not, is bad? When working correctly the pedal is high and firm and the truck stops very straight and very well.