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I think it is a good idea to be able to disengage the front axle when in low range. I do not use low range much in the M939's, but I do not put it in reverse when I do.

Yes the larger one has the spring brakes and a service side. The smaller one just has the service side. If you load the P2P Softwhare here on SS it will help you understand how the system works. http://www.steelsoldiers.com/showthread.php?79228-TM-s-for-the-939-series-trucks The M939 MV truck...

On each of the rear axles there are service brakes and spring brakes. The bigger ones are the spring brakes, used for the parking brakes.

You can down load the P2P software here on SS and it will help you trouble shoot the problem. I used it a lot when I had a brake problem.

It maybe the treadle valve or one of the check valve.

I would think it maybe a line that is leaking. I would turn the truck off and push on the brakes and see if you can hear a leak, also get a soldier B to help out. FYI, if your truck has a ball valve on the front service glad hand, Do not close it, it will cause brake problems. I know first hand.