m4gnum

Hi. I wanted to change the coolant in my 2015 Mercedes C300 W205.

To drain the coolant I followed these steps:
- opened the coolant reservoir cap
- removed the lower engine cover,
- connected a hose to the drain outler,
- opened the knob and opened the coolant reservoir cap
- drained the fluid and squeezed the hoses to drain whatever is left

Then I removed closed the drain knob, removed the hose, and proceeded to refill the system using a kit purchased through Amazon
- screwed the attached cap from the kit
- attached the vacuum part from the kit
- connected an air compressor
- ran the compressor to about 90 PSI
- opened the lever to start the vacuum and the gauge showed -40 bar and all hoses collapsed, it wouldn't vacuum any more
- when I closed the compressor port, it was losing vacuum little by little down to -20 bar over 5-6 minutes
- I inserted the fill hose into the coolant jug and opened the fill lever until the gauges showed 0
- the system took around 0.7 gallon of coolant which didn't seem enough so I closed the fill lever and opened the vacuum lever for a few seconds and then closed it off and opened the fill lever
- it took another 0.3 gallon, so a total of 1 gallon and the coolant reservoir was filled up to the plastic part which is good

Now, the manual stated that it takes 2 gallons but it only took 1 gallon. I checked for any leaks but couldn't find anything, despite the vacuum on the gauges dropping. I took a roind trip around the neighborhood and the engine temperature stayed within ~90 degrees, the coolant level did not drop.

Is that normal? Why would it loose vaccum with no leaks and why would it only take 1 gallon of coolant?

I also noticed that is has a secondary coolant reservoir which I guess is the "low temperature coolant" according to the service manuals. Should this be drained as well? Does the car take a total of 2 gallons, so 1 gallon in each of these cooling systems?

I would really appreciate your help since I cannot find any information online.