Cat C15 1,100k miles pressure started buld up in cooling system

Have Cat C15 on 2003 Columbia and 1,100 m on it runs great 6.9-7.5mpg,and it started build up pressure in cooling system,I have had to add in 500m 1-2 gal of antifr.did check in shop using dark blue stuff putting in place of cap,see if gas from broken head gasket would change to yellow and it did not.Replaced also air compressor,still does.Don,t know what to do,need input from you guys.No water in oil.No oil in water.

 

Thanks for reply.I did replaced cap.after driving 15-20 m got back in shop and easy to see pressure more than 15 but cap looks like locked up unless you start open it to dump pressure.before it was pressure just little,and antifreeze was at level all the time.

 

thats the problem dont see any where leak but it some how goes out or may be in.when engine started no white or any smoke,i,m not sure but looks to me it goes out trough relieve valve on cap,not gas but antifreeze it self.

 

I will do that tomorrow,oil is clean,antifreeze is clean from oil.on idling it does not build pressure.checked in shop with special dark blue liquid on top of surge tank,if gases in present,should become yellow,but it does not change color. My 1st reaction on it was broken head gasket even engine was never overheated/i,m o/o and no body was driving my truck from new.and it happened before like-2years ago I replaced head gasket and put new plate on,resurfaced head it self...