I noticed about 6 months ago when moving the coach that our Transmission retarder was acting very inconsistent. Sometimes it would kick in immediately when releasing my foot from the accelerator pedal and sometimes my bottom air pressure gauge would drop to the point, I would get a low-pressure alarm but would build back pressure when I applied pressure to the accelerator. I can manipulate when the retarder actually kicks in by applying pressure to the gas pedal and then releasing it until the retarder kicks in once again. When it does my air pressure builds back up and maintains. When I turn off the retarder I no longer loose air pressure when applying the breaks. When parked we have allowed the coach to air up and then turn if off and walk around no air sounds and the pressure holds.
We also had the transmission serviced at Stewart & Stevenson in Albuquerque NM and he said that the engine did not seem to be communicating properly with the retarder. We then took our coach to John's trucking in Tucumcari NM who just rebuilt the engine and he said he could not find any issue with the communication between the engine and the transmission. We are now going to take the coach to Massey's in Phoenix for an oil change and service and will have them look at the system. Does anyone have a recommendation as to what our issue might be? We have a Cummins ISL 400 and Allison transmission.
From what you are describing that I would think you are having trouble with either the air valve
or the actuator for the retarder is sticking and losing air.
Agree. While an earlier coach (1998), this post describes the same symptoms Allison retarder accumulator failure (https://www.foreforums.com/index.php?topic=42026.0)
Mike has a great write-up on cleaning and servicing the valve Allison retarder air valve (https://www.foreforums.com/index.php?topic=31477.0)
There are also several posts on replacement options, most from eBay. Let me know if you need links to them.
Out of curiosity, when was the air dryer last serviced? Any signs of white powder when you open the tank drains? A failed desiccant cartridge that let desiccant into the air system could definitely clog the accumulator air valve.