On a '97 t-600 with a red top N-14 we are hearing the air dryer "spit" about every 30 seconds while going down the road.  Air pressure has risen about 10 lbs and now stays close to 130.  Air pressure never drops unless you pump the brakes several times, then it will go down but comes right back up.   While it is pumping the dryer will not spit, only when it reaches the 130.  Is it right the dryer will only spit when the compressor cycles off? 

We have replaced the governor on the top of the compressor and  put a new cartridge on the dryer.  I have heard of some poppets in the system that might be hanging?   What else should I be looking at?