These are parts for a 944 master cylinder. apparently they do not supply any for a 928 master cylinder, only complete units are available.

The brakes work but the pedal always goes slowly to the floor.

It passes the booster test-pedal depresses slightly when starting the engine, holds, but then starts to sink slowly.

I ran a test in the service manual that seems to indicate the master cylinder is "Bypassing"

Is there any other way to verify this "bypass" function and what else should I look at as possible sources of the problem?

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If the brake pedal slowly sinks to the floor under light pressure, there are really only two possibilities: Brake fluid is leaking from the system.
The master cylinder is "bypassing".

If the system is leaking, there will eventually be evidence somewhere. Find the stains, and fix the leak.

"Bypassing" will show no external leakage. The master has two flared rubber cups pushing on pistons inside the cylinder. If the walls of the cylinder become worn, or, more commonly, corroded, or the flared cups become  hardened or worn, brake fluid will leak around the edge of the cups. Normally, the leakage will be worse with very light foot pressure, and if you push harder, the cup will flare more and seal against the cylinder.

Rebuilding the cylinder yourself works well if the problem is hardened or worn cups. It does not work well if the cylinder is worn or corroded.

Wally Plumley
928 Specialists

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Here's a nice guide about the brake cylinder repair. Brake_servo_guide.pdf

I'd give it some thought and consider buying a new one as braking is a life saver. You want that to be reliable.

Theo