Truck Repair: Brake pressure/ overheat?, purge valve, cold climates

Question
Kenworth T300 with a caterpillar c7 , 250hp with 310000 miles, year 2004.

Hi, above is my truck's info. When I start my truck, the brake air pressure is really low and make noise. Is something leaking?

Also, when I drive the truck for 15 min, the temperate gets high to 210 degrees n engine fan comes on, but it drops to 180 on highway, and 210 on local. The engine stays on, is this normal? When the a/c is on, then the engine fan turns on n temp drops to 180. Is this normal? What should I replace?

Thank you,
Garry

Answer
well if your temp is dropping then your thermostat is working correctly. Your temps are going to be higher in the city because your not getting alot of airflow through the radiator. Your thermostats are 180 when your temp gets too 210 the fan kicks on the fan is going to run until you get back to 180. when you reach 180 the fan should kick off until it reaches 210.  When your a/c is on the fan is going to cycle on and off to blow air across the condensor to cool the refrigerant in your a/c system. Everything sounds normal to me. If you have down time and the money wouldnt hurt to do the thermostats at the milage you have on the engine your not too far off from when they start to fail.

When you start your truck if the tanks are empty and you hear this hollow ringing sound thats just air vibrating in your tank as they build pressure. if your hearing this psst sound every couple seconds then you have a purge valve or unloader valve leaking on your air dryer. If you don't check for moisture in your tanks i would open the pet cocks of your tanks when you have air pressure in them if any moisture comes out i would get your air dryer cartridge replaced to much moisture in your system will one make valves freeze in cold climates and two make valves go bad because of the water.