Draining Air Tanks in Sequence (Check Valves)

Yes, completely draining the wet tank first will test the check valves in at least your primary tanks and usually your secondary’s.
The difference, some older trucks fill the primary tanks to 60-90 before filling the secondary’s. Most newer trucks ( at least what I’ve seen) fill primary and secondary at the same time. With the newer version you can test both check valves by draining the wet tank. As for the older system, I’m not for sure, the farmers that owned the ones I’ve seen, didn’t worry about the air system until the truck wouldn’t move.....

 

Can you explain this further - wet tank?

I’m about to learn something - seriously.

 

When when compressed air is cooled or is able to expand, it drops water. The air system starts with the air compressor which pumps air to the air dryer, the air dryer under normal conditions should remove 99% of the water. From the air dryer it pumps to the first tank in the system (wet tank). It’s the first line of defense if your compressor creates more water than the dryer can handle. From there, the air is transferred to your primary and secondary air tanks, depending on the year make and model, this can happen in a lot of ways. But DOT mandates that there should be check valves so that if one air system fails, the other is there to get you stopped.

Some newer trucks have a wet and secondary tank together, it looks like one tank, but there’s a drain on both sides. There is a divider in those tanks with an unserviceable check valve.

 

Here is a diagram google pulled up from this website: Basic Air Brake System Schematics
This shows the basic air system that the truck brakes are based off of. ABS,ATC, and RSS are just computer controls and valves added to this system. If the area in grey doesn’t catch the water, you get freeze up conditions in below 0 condtions. This can happen most commonly if your air dryer needs serviced or your compressor runs to much to maintain the air pressure leaks.
So... if your wet tank is dry, there shouldn’t be water in your primary and secondary’s, and if you have water in your system fix the air leaks and service your air dryer. I’ve found you get what you pay when buying cartridges for your cartridge style air dryers, in my case, if it doesn’t say WABCO on it, it doesn’t work. TRP and Alliance have both let me down....

 

Great explanation, I do have a question about the last part.

If both air systems fails, would you not be able to stop, or would you not be able to release the spring brakes and the truck would make a dangerous unwanted stop?

Thanks in advance.

 

If the primary air tanks fail, your only steer axle brakes become inoperative and the park brakes on your drive axle set. Manufacturers believe (and it holds some truth), that it will be a slow release. The the park brakes will set slowly, easing you to a stop. But we all know the yellow park brake valve will pop out lower than a certain PSI (usually between 35-45). The Idea behind it, is the driver will feel the drag on the truck, the engine working harder and stop before that happens (because the park brake starts applying at 60-70psi). Furthermore they aren’t planning on both failing suddenly.
This diagram doesn’t show the trailer which is completely operated from the secondary system. Since most all trailers on the road have park brakes, they are designed to to start applying when the secondary air drops below 70-90 psi and the same thing applies, driver feels the drag and stops to investigate before the sudden application of the park brakes. Which makes the point, make sure all (both truck and trailer) brakes are adjusted properly so your park brakes will stop you!!!

If the entire system fails, you probably won’t be as worried about the sudden stop as the Toyota you probably just ran over!!!