I couldn't find anything before posting, so I apologize if this was answered already 

I bought a 96' Tracker 2dr convertible, had bad brake lines.  It had sat for quite a while, so I replaced every single brake line on the truck.  Both steel and rubber.  I then discovered the master cylinder was bad, so i replaced that.  I power-bled the entire system.  Now, if I pump the brake, I get great pressure.  But, if I stop pumping for a couple seconds, and hit the brake again, no pressure at all, and the pedal goes to the floor.  If I keep pumping, it will build great pressure again.  I triple checked for leaks, and there are none to be found.  The front calipers seem to be moving around, so they aren't frozen.  I haven't taken the drums apart yet though.  Would anything in the wheel cylinders cause a problem like this?

Thanks guys!!!

Sounds like you might need to bleed the brakes again, and check
the back brakes, they might need adjusting, and could cause
the issue you are having

Wild

Could be leaking internally on the booster.  I have heard that they can leak internally and you'll never see a drop of brake fluid.
But I would take wild's advice and make sure you have the brake system bled good first.  Try just doing a simple gravity bleed, instead of a power bleed.

I bled the brakes again for close to 2 hours this morning.  The pedal still feels the same, and no air came out.  There's no fluid leaking anywhere, and the reservoir is still full.  I did notice that when I'm driving it, the first pump can lock the rear tires up, but the front brakes don't start working until I pump the brakes a few times.

If you ever ran the reservoir dry, during your brake swap or brake bleeding, maybe you need to bleed the master cylinder.  Even when I have ran it dry, I have never had to bleed it, but I hear some people do.
In what order are you bleeding the brakes....which wheel do you do first, second, third, and last?
Have you tried bleeding the proportioning valve?  Again, I have never had to, to fully bleed the brakes, but some people do.

The proportioning valve should prevent an easy rear lockup, but if the
front brakes aren't working right away, the calipers could be hanging and
need to be rebuilt or replaced.

The first brakes to activate should be the front, since they don't pull
back from the disks, the back on the other hand have springs and do
pull back away from the drums.

I still think you need to adjust the rear brakes tho, could be a lack
of pressure from them being out that makes the front not work
right away

Wild

Alrighty, just got back inside.  I pulled both the rear drums apart, checked the wheel cylinders, and cleaned out the adjusters.  Made sure everything was moving easily, adjusted them, and put them back together.  Still the same issue.  I'm thinking that the front calipers are frozen, like Wildgoody said.  They look to be moving when the pedal is pumped, but I think it's just the caliper flexing under the pressure.  Calipers are cheap, I think I'll just put a new set on and go from there. 

Oh, and the pedal has to go to the floor before the rears will lock up, but they have a good feel when I'm pushing the pedal down.  They gradually stop more as I put the pedal down. 

Would a stuck caliper push all the pressure they've built up right back out?  I've done many brakes in my day, and I've never seen this 

The hanging calipers just need to recoil back a little bit and the pedal
will have a lot of free play, if they are cheap, replace them and bleed
the calipers.

Wild

New calipers are on.  The old ones were smoked, and it took my air chisel to remove the pins.  But, still, the same issue.  I did notice the proportioning valve in the rear has a bleeder screw.  Maybe I have a pocket of air in there?  I also noticed it was a bit wet.  Nothing dripping, but could be a potential bad leak in the future.  Anywhere to get a remanned one of those?  Looks like it's adjustable by the height of the rear axle

Not sure if your '96 has the same system as my '92.....

I had a very similar problem, but not quite as bad.  When I held the pedal at a stoplight, the pedal would slowly sink.  I went through the booster and master cylinder, without curing the problem.

My '92 had a pseudo anti-lock circuit for the rear brakes only.  The front brakes always locked up quickly on wet or slick roads.  I removed the accumulator underneath the master cylinder, and the distrivution block next to it, and ran the lines directly to the master as a test, to see if it made a differnce.  It was actually really simple - just had to bend the existing lines into position once the bits were removed from the circuit....  My brakes have never worked better.

Not sure if your car is similar, and I'm not going to advocate modifying braking systems.  I'm just sharing my own personal experiences.

Also - I was told that a new master cylinder MUST be bench-bled prior to installation - bleeding the master on the car supposedly will not evacuate all the air from the bore.

Looks like the new calipers did the trick.  After bleeding the LSPV, I bled the rest of the system again, and air started pouring out of the front calipers.  Just took it for a spin, and I have rock solid brakes.  Thanks guys!  Now, to weld up the rockers...  I can see through them!