Spongy Brakes Question

[LSUMurse]

My brakes have become spongy, to the point that my dash brake light comes on. Most of the time, as I come to a stop at a stop light, they perform so-so and stop the truck. As I am sitting there, the brake pedal slowly depresses further and requires more pressure to keep the truck from moving. Sometimes it goes straight to needing the aforementioned amount of pressure with no stoppage in between.

I have converted the truck from manual to power brakes, but it didn't start doing this until after I put a new engine in her. I did not break the system, I unbolted the master cylinder from the booster and pulled the MC over to get it out of the way.

I went to bleed the brakes today and didn't get much force from the bleeder valves, mainly just a trickle. I never noticed any bubbles. The passenger side rear valve was gummed up so bad I had to remove it and basically drill it out.

I pulled all the drums and did a rudimentary inspection of the system. I didn't notice anything that looked wrong, though I am not incredibly familiar with drum brakes.

Should I open the lines at the tee in the rear and see if I am getting good flow? Maybe something dislodged and stopped up the lines? I'm not sure where to go from here.