I'm stumped, what's the primary cause for hot water from the boiler not entering the grouphead when the lever is lifted? I perform my own maintenance, but in the last day or two, the pre-infusion draw has yielded very little water and when it comes to the actual pull, there is little resistance to the pull until the last half of the pull - thus yielding about 50% less espresso.

Upper Group Sleeve - no obstructions or cracks, good gasket, ~4 years old.

Piston gasket seals? These are about 9 months old and properly oriented / \
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I've disassembled the grouphead twice without resolving issue.

What's the inside of the boiler look like? Could you have gotten some kind of scale or other kind of obstruction in the dipper tube? That's what it reads like.

Do you have the plastic piston? They can unscrew from the rod at times and restrict flow.

The piston gaskets should be oriented like this:

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Before my machine kicked out it also drew little water to the group.
The piston seals were bad and in my case, the sleeve was cracked and the sleeve seal was frayed. i doubt it was ever lubed or maintained for that matter.

I would also check boiler pressure. You could have this problem if it isn't fully heating.

All good advice above.

I'd recommend pushing out the piston and making sure the seals are oriented correctly. Katzer has it right - the flared end of the upper seal faces up, and the flared end of the lower seal faces down. Then while it's out, stick a screwdriver shaft through the piston rod hole, and grasp the piston with your strong hand and make sure the piston is screwed tightly onto the rod as far as it will go. Especially for the plastic pistons this is a likely cause of this problem.

Scale blockage is less likely, but sometimes happens. Check the dipper tube as Tom said, and If you have the plastic sleeve you may want to remove it to make sure there is no blob of scale up inside the group where it might block flow into the inlet channel of your plastic sleeve.

A last unlikely possibility is very worn lever pins. A lot of slop there can cause the lever yoke to hit the topnuts before the piston is fully raised.

Edit addition: Also, make sure you haven't re-installed your lever upside down. Is very easy to accidentally do that.

The post above addresses feeble water delivery, which you would notice when raising the lever and flushing water without a portafilter in the group.

If you are getting good water delivery from the group when you raise the lever with the portafilter out, but are getting unexpectedly low shot volume, that's a little different issue. Besides making sure the seals are good and oriented correctly, and the lever is not upside-down, I'd look closely into the temperature. A cooler group and/or cooler (lower pressure) boiler will tend to give you lower volume shots. If you're updosing and pulling shots with less headspace, that should also lower your volume.

A big thank you to all of your responses.

It was the piston gaskets. They were incorrectly oriented with the flared sides facing together than facing away. Good catch..."Katzer has it right". The incorrect orientation, IMO, caused the gasket/seals to create a flat polished surface over time and affected the water draw. A pair of previously used properly oriented gasket/seals definitely showed a different profile.

I appreciate the timely and helpful responses!

Just a brief follow-up.

I wasn't sufficiently happy with the amount of water flowing into the group head after lifting the lever. Following the remarks from Crazy4Espresso regarding the plastic piston experience, I *carefully* examined the shaft and in this case the brass piston position. Using a rubber kitchen glove and a Phillips screwdriver, I was able to seat the brass piston to the shaft after 1 full revolution. That was enough to clear the sleeve's water inlet. The flow into the piston face is now unrestricted!

Part of my troubleshooting process included removing the group head and lever from the piston shaft. With the piston in the normal down position, I blew air into the group head inlet pipe and moved the piston up until the air freely entered the sleeve. Did this several times to get an accurate position on the shaft. Marked the shaft with a Sharpie and aligned a piece of blue tape on the chromed portion. Removed the piston and shaft, tighted/seated the shaft to the piston and reassembled. Blew air into the inlet and noted unrestricted air flow about 1mm to 1.5mm before the marked reference points on the shaft and grouphead or about this distance =. So, in conclusion, make a special effort to seat the shaft into the piston by hand (very unlikely you'll strip the threads). It may appear to be seated with crud surrounding a twist of thread, but it is enough to make a big difference.

I'd be curious if someone with an ME background can explain the dynamics of a vertically oriented brass piston rotating 1 revolution on it's axis over a period of 15 years where the piston is lifted and lowered 6 times daily.

Thanks again for the feedback to this thread. All the tidbits helped!

Sounds like you found your problem.
Another thing that can cause poor water flow is worn lever pins. I had to replace mine on my 2003 in order to restore a good gushing flow of water.