brake problem...

[Longhorn Man]

I got a call from GMC Junkie recently on her son't truck, and was unable to help.
69 I believe, 1/2 ton, 4 wheel drums.
His pedal was going to the floor, took it to a brake shop, and they replaced the master cylinder. They checked everything 2 or 3 times, to no avail... The problem is, no matter how much they bleed it, they are constantly getting air in the stream coming out.
Remember, this is a brake shop, and no matter how much we wanna laugh at the chain stores, lets admit it, for the best part, they know what they are doing. They were checking lines, looking for leaks, looking at the wheel cylinders, everything.
They ended up giving it back not working, and charging a cut rate (probably only for the master/labor to install).
So she had it towed to her regular mechanic. He has been turning wrenches for her for quite a while now, and sounds like he always does her right... he called her today and was saying the same thing.... can't figure it out, makes no sense.
Her son (junkie jr)is talkin crazy talk... like sellin his rig.
We can't let him do that. He's one of us... just real quiet.
Can anyone throw out some sugestions? Everything I mentioned, he already looked at, and like I said, her regular mechanic sounds like he knows his stuff... she never has anything to say bad about him.
We are talkin an all stock 69 or 70 1/2 ton chevy.... 4 wheel drums, unmodified for the best part, I am almost poisitve it has manual brakes (no booster).
Anyone have any ideas?
Jodie should be getting on here this evening and answering specifics that I didn't cover... she just wanted to get this up on here as soon as possible and she's at work.

He has had some damage due to a drunk driver, but it's been 6 months and driven most every day.

I don't think I see a boosterr in there... I am 99% positive it is manual brakes.