85 W900 Parking brake

I've never seen that setup before. My 1980 W900 has the 3 buttons.Pretty sure I have to charge the system with the yellow knob, then push in the blue button before the park brakes on the truck will release.

 

Yeah, you have to push and hold the blue button to release the spring brakes, you have to hold it down it doesn't stay released

 

On mine nothing releases with just the yellow button in. I actually have to charge the red and blue valves by pushing in the yellow valve first, then I can push in the blue valve to release the truck brakes and the red valve to release the trailer. I don't have to hold the blue one in it stays in on its own.

 

Something is definitely off.. They are all independent or should be.. the Red is your emergency / trailer supply valve.. you ought to be able to push it and it should simply supply air to the trailer only...The yellow is basically the parking brakes, you should be able to release in without it effecting the trailer in any way. Some tractors, if you have both red and yellow applied, the both will pop if you pull out the yellow.. but they should release separately. The blue is really something that just sits there nowadays.. most trucks don't even have them anymore

 

I think even my dad's old '78 W900 was the same way too.

 

Mine is same way too. Yellow supplies air to blue (tractor) and red (trailer). Need both yellow and blue pushed in to move it.