Brake booster
 

Brakes rock hard on startup and drop to the floor after engine is started, this happens over night and ive a vacuum leak on the #2 bank. Questions being on average whats soec psi for the vacuum system? I get 18-19 psi when idling on the booster hose. 2 if the booster is leaking is there a test i can do?

Yes, as far as a test
Start engine and run for 20 seconds or so to charge the booster with vacuum
Shut off engine, count to 10 slowly
Press brake pedal down as far as it will go 3 times, should get 3 easy to press pedal pushes until booster vacuum is gone and pedal gets hard to press
If you only get 1 or 2 then its leaking

The reason for the larger size of a power brake booster is so it can hold a reserve of vacuum in case engine should stall while driving, it would allow driver to pull over and stop safely

If pedal is going down too far with engine running you may have air in the system or out of adjustment rear brakes
Hold out the Parking brake release handle then pump the parking brake pedal until it doesn't go down farther, you are adjusting the rear brakes doing this, normally this is done by using the parking brake daily or backing up and stopping using regular brakes
People with automatics often don't use parking brake
And when backing up they just shift to Drive without pressing down hard on the brakes to stop
So rear brakes never get adjusted