Keith, the first question is your air system building up to 120 PSI and than hearing air dryer pop off.
Second question, when you pull yellow button out, to apply parking brake does the air at valve ( one
yellow button is pulled ) seems like the air takes longer to stop hissing sound. Third air pumped up and
air dryer poped off, with engine shut off does the pressure gauge on dash go down and is it more than
( 1 ) pound per minute. The answer to above questions will tell a person if you have a problem or not.
The first question you have to have min 120 PSI when air dryer pops off, answer should be yes.
Second question applying park brake ( pulling Yellow Button out ) hissing sound is longer than normal.
answer be no, if answer is yes you have a pancake bad on your spring brake on rear axle.
third question with air supply pumped up and air dryer pops off at 120 Psi, watch pressure gauge on
dash if needles are going down more than ( 1 ) pound per minute. this will give a idea if there is
something going on with vehicle air system. Dave Atherton Retired Cat Mechanic