Brake MC/Hydroboost question

FNG here. Long time lurker, first time poster!

I have a 2000 Chev 2500 Silverado with the hydroboost assisted four wheel disc brakes. With the truck off the brake pedal holds firm with light to heavy pressure. With the truck running the pedal feels normal but with moderate pressure it can be pushed to the floor.

All my searching says how to diagnose the hydroboost but nothing says anything about my situation as an outcome . So I'm here for some opinions on diagnosing, the master cylinder, hydroboost, or power steering pump, or.........?

FNG here. Long time lurker, first time poster!

I have a 2000 Chev 2500 Silverado with the hydroboost assisted four wheel disc brakes. With the truck off the brake pedal holds firm with light to heavy pressure. With the truck running the pedal feels normal but with moderate pressure it can be pushed to the floor.

All my searching says how to diagnose the hydroboost but nothing says anything about my situation as an outcome . So I'm here for some opinions on diagnosing, the master cylinder, hydroboost, or power steering pump, or.........?

your problem I think is a very small leak...if checking all calipers and finding no leaks and you have no loss of fluid then this may be the ABS valves leaking "INTERNALLY"...It does occur when they get old and have lots of mileage..

the engine running you have much more pressure so the fluid will leak..
the master cylinder may be worn but I would think that this would be the same with engine on or off...