Yeah, the preload is normal and accepted in hydraulic systems. I had exactly the same question with the same concern when I did mine.
Here's where I learned about that in my build thread:
Originally Posted by Rope-Pusher
You said that the slave cylinder preload spring will extend the pushrod all the way to the end of it's travel if nothing stops it. Doesn't that mean that it is pushing the release lever until the bearing is tight against the diaphragm fingers?
On any HCRS, when the slave is preloading the bearing and the clutch pedal is at it's upstop, the pushrod of the master cylinder should be fully extended, so that the piston in the master cylinder uncovers the port in the cylinder wall that leads to the hydraulic fluid reservoir. Then the fluid gravity feeds into the now unpressurized forward side of the master cylinder and fills any voids in the line. When you push on the clutch pedal, there may be some clearance in the connection between the pedal and the MC pushrod, and there will be some amount of pushrod movement before the MC piston moves far enough to block the fluid reservoir port before you can start building any pressure in the HCRS line in order to start building pressure at the slave cylinder to start to extend the pushrod there and start releasing the clutch.
Rope-Pusher is a retired engineer from a major auto manufacturer's manual transmission division.
HCRS = Hydraulic Clutch Release System (engineer-speak for hydraulic clutch).