INTERNATIONAL: electrical, drive train, brakes

[Kenny7]

I'm new here and id assume this is the correct place to start with questions.
first off i bought a 1993 international school bus thomas built body 72 pass. license plate was last registered 2013 so it sat for about 6 years. it has the DT360 with a spicer 5 speed the owner of it always started it up and let it run for an hour about once a month and it starts right up no hesitation at all thats good. but that was all he did so i went to hit the brakes and the pedal dropped and brake alarm i ended up replacing most of the (hydraulic) line and gravity bled the rears because it has a hydraulic pump that goes off when you hit the brakes. anyways was that correct way to do so? brakes feel like a normal car now and no alarm. i checked engine oil it was full and doesn't look bad at all i think it'll be fine. i don't know how to check the trans oil but id assume its fine... oh and the parking brake does not work at all can it be left in gear? (again idk i've never owned a diesel or a heavy vehicle before) one thing ive noticed upon start up is that there are no dash lights that'll come on like a regular car when starting just the gauges will sweep i was told lights should come on... and the last thing is it starts right up but it seems to be in the low end volts would that indicate that it needs new batteries? id assume not since it actually starts right up. ok thanks