I have a 1988 chevy s10 with extreme air in the master cylinder

yes I did very easy process

I might add that in bench bleeding the master cylinder, I was able to get all air out. in thinking that I might have a bad MC I removed it from the vehicle and manually bled it again with success. Put back in the truck and same thing air in the MC.Im thinking maybe a bad proportioning value.

Not for spongy breaks. If there is ABS system on there, the ABS may need to be hooked up to a computer/ scanner to run a program to bleed that out. You just can't do it like normal by opening the bleeder valves.

An 88 won't have abs. Nor should they be hard to bleed. I've bleed several trucks of that generation. 2 things, I have never seen creamy brake fluid unless it was contaminated, maybe i just havent seen one bad enough. Second tighten everything up and pump the shit out of the brakes then check the wheel cylinders and caliper for any moisture. And see which reservoir is getting low. Also I've wanted to place proportioning valve before and all of them were never bad. Idk if I've seen a bad one. Replaced more due to bad threads than actually bad

Andrew brought up a good point. Kinda over looked what you said here. Yes do what Andrew says about checking the wheel cylinders, check for leaks, But now I'm thinking that the break fluid is so contaminated that you cannot get a solid pedal. I would make sure all the break fluid has ben flushed out with new fluid.