3406B building to much air

I'd try to get it to replicate the audible air leak by driving, then stop immediately and try to find.
As for the high air pressure, you might be able to lower it by adjustment in the governor. On a Bendix D2 governor, you just unscrew the black plastic cover (turn counter clockwise by hand-but might need to initially break loose with pliers), loosen a locking nut with a deep socket (7/16" I think), & then turn the adjusting bolt that has a groove for a flat head screw driver. I can't remember if clockwise or cc lowers air pressure. I do remember you don't have to turn much to adjust, like maybe start off turning 90°-180°.

 

The air leak was probably the safety valve on one of the air tanks. Usually they pop in the 150ish range.

Unbolt the governor from the compressor and make sure the unloader signal is getting pressure at or above 120-125 ish.

You can just shut it off and unbolt it when it's at 130 and see if there is air coming out of the hole in the governor that goes to the unloader.

If it is, the unloader could be an issue, if not, either a governor issue or air supply to the governor.

 

Those old stratoflex lines are notorious for leaking, causing frequent cycling of the unloader and air dryer popping.

But, if you remove the supply line to the governor and the pressure seems good, I'd probably throw a governor at it and that'll likely fix it.

 

IIRC, it’s the line closest to the block on yours. RES? I think is what it's marked as.

Since you have no air at any time while it's unbolted, only one of them is going to have pressure

 

Times 2 on the “Overloader”. Had a similar issue on my 1994 E model years ago. When cold it would go beyond 120, once it warmed up or cycle 3-4 times, it would run/pop back to normal 120. It was the overloaded kit on top of the compressor. Very easy to replace. But they had 2 types, I couldn’t tell the difference today.