Freightliner Classic won't build air

are you sure it is compressor issue? You may have bad/worn 4-way valve (if used) ore air curcuit valves (may be installed in front bumper).

What air curcuit looses air? First ore second? do park brake buttond pop up when you press brake pedal several times?

 

There are two types of air systems used now. The most popular - four way valve. One steel tube from compressor and four outer lines for every curcuit. First - for rear brakes, second - for front brakes, third - for park brake valve and trailer air supply, fourth - for cabine air bags, air controlled transmissin (ZF), clutch servo (if equiped), and all other additional equipnet, like PTO solenoids, fan clutch solenoid and other.

Air suspention is supplied separatly with its protection valve, becouse air bags require higher pressure.

The older system is with separated circuir valves: evey valve for every cirquit. I'm not sure about Classic, but these valves are in bumper on Century class. And there is hidden valve inside air tank.

50 PSI is too low pressure. 100 (6.5 BAR) is enought. There are two air pressure gauges in cabine - for first and second circuits. What circuit looses air?

 

if your primary will not build air but the secondary does, then your check valve in your primary tank is clogged. you can replace this valve.

 

last year I got similar problem with old DAF 95. We concidered problem is in first tank valve, and we replaced it with new and problem did not go away.

The problem was hidden in second tank valve (front brakes). There are backstream valves inside that let first and second tank share air when pressure in one of tanks is lower four bar. Backstream bore was clogged and compressor was off till second tank pressure was lower the compressor actuation point.

Changing both the valves solved problem.

Newer vehicles are with integrated system like air driyer and four way valve. Some of them are with fully electronically controlled air system (like MAN E-cam).

http://www.wabco-auto.com/products_wabco/air-management/air-processing-unit-apu-wabco

And sometimes when air system is really old and worn we did conversion. And it is sometimes cheapper changing old valves one by one. Sceptics say, new air processing units are too expencive, but Wabco supply rebuild kits and it is really easy to repare it. And I'm sure this way is better in safety! As I can see, when one valve fails, the other fails soon. Valves issues are usually related with damaged with oil rubber parts inside valves). I always say to owner when air drier is bad ore compressor sucks oil in air that it is risk that not only one valve goes bad. I tell him to change air driyer with air four way valve, trailer controll valve and foot brake valve. Sometimes I use to change ABS valves, sometimes I change park brake valve.

So I wanna tell you that if you see that one valve fails check comoressor and air dryer. If you see oil in air - rebuild your compressor till you get more problems.