Brakes FUBAR on 78 chevy 4x4, help!

hey all,
I need some help guys. I have a 78 (actually my dad's daily driver) 4x4, about 6 months ago, the trucks didn't seem to be as strong as they had
been(used to stop on a dime) anyway, you had to put the peddle all the way to the floor to make it stop but it didn't even feel like it had any power to it(it has power brakes, btw) anyway, we replaced the proportioning valve, didn't do anything, put new pads on it and it didn't help either. We ended up tightening the rear drums up to make it stop, and it will, but it doesn't have any of the power it used to. The front calipers are touching the rotors at least (vibrates, needs to be turned) but it's like it has no power. I've also replaced the master cylinder and bled the lines, still the same. Could it be that power booster? It will stop, but you gotta put some pressure on that peddle, it used to be you barely had to have any force on it to make it stop(all old chevies i've ever drove felt like that) Anyway, is there something yall know of that might help? I think it has to have to do with those front calipers because you can't feel the front end bite down like usual when you stomp them to come to a quick halt. Any help yall can offer would be really appreciated! Also, it has new front pads.
Thanks! TB
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way
valve,
ended
doesn't
the
some
to
down
If it's taking excessive PEDAL effort to stop the truck (pedal is hard to push), then it's most likely the booster.
Doc

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It's not unbearable hard to push down, but it's much harder than most 87-down chevy trucks i've ever driven. Lol, it's about the only thing i have left i can replace!
thanks Doc! TB

4x4,
power
on
Anyway,
can
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Does the truck pull to the side at all. Which tires slide on slipry ground?
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Tell me if I'm all wet on this one, but to test the power boost unit, stop on the brake pedal several times to exhaust the "stored" vacuum, then start the engine with your foot on the brake pedal.. If it drops a bit further, it's working ok.....(I think)
On an older mercedes, I had a simular problem (acting like air in the lines that would never bleed out), but what had happened the rotor were worn more than the limit and the calipers pistion were trying to push out too far....new rotors fixed it....MBZ gave only ONE mm of wear on my model
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i've never noticed it slideing one way, but the truck stops dead center; don't pull at all.
thanks! TB

ground?
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Try sliding tires on slipry ground. Make sure all four are grabing. Then go from there.
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