Air break loss while driving

You probably have air leak. Air leaks out pressure drops. Air compressor come on fill tank back up and then purges.

 

If you have air ride that will use some air.

Air dryer should purge every time the compressor kicks out at 120psi. If the dryer is purging a lot it can mean you have a leak and your compressor is cycling a lot or it can just be your dryer needs servicing.

 

Chock the rear wheels on level ground, keep engine off and release the brakes. Walk back to the rear axle and listen for an air leak. May be in the frame rail or a maxi leaking, but you should be able to hear it…

 

First - TAKE YOUR EMAIL OUT OF THE POST.
Everything I am posting is based on the accuracy of your first post.

IF you have posted this right, with low loss and a 15-minute purge cycle, then ... THIS IS NORMAL. NOTHING IS WRONG ... The next quote is why.

The age of the truck and being an international truck, there will always be system leaks as the truck ages, they use plastic fittings and there is an age factor to them.

I would worry if you lost 10 psi in less than a few minutes just idling, but 4 to 5 is nothing.

Yep, that is how it is supposed to work.

 

Yes, owners neglecting trucks is normal to loose air, My 27 year old truck will go a hour and 20 minutes between purges and holds 60 psi over a day off, start at the governer (where it actually mounts) and start spraying your way back with soap and water, throw a cartridge on or even a whole dryer if it’s one of the cheaper ones first though

 

Every 15 minutes on the highway isn't horrid. It's moving up and down, side to side. Do you have air bag suspension?

 

For what it’s worth. Air leaks often get worse or first noticed when the outside temperature drops.