What would cause a caliper to not release when the pedal is not being applied and the opposite side appears to work normally ?
The side in question has a new rotor and caliper .front brakes

 

Could be an obstruction in the line but more than likely it is not properly lubed, bled, or there is something preventing it's return. I would pull the caliper off along with pads, reinstall everything after lubing it up with caliper grease (no need to disconnect brake line unless reinstalling doesn't work). It also could be a failing master cylinder but then the issue should be @ all 4 wheels. If it doesn't work properly after reinstalling and bleeding the line then you may have a defective caliper, not unheard of as most "new" calipers for our trucks are just reman's.

 

The caliper was installed a year ago when the original rotor was cracked and would pull when braking.
To install pads this time, I could not back the piston until I removed the brake line . It then moved easily .I believe I will have a cracked rotor again if I do not find the cause . I get good flow when bleeding . I wonder if the caliper slide bolts are messed up . I did lube them and they did not look bad .

 

Your rubber line to caliper is most likely bad. With age they collapse inside. If the caliper releases when you loosen line and then retracts easily replace your rubber line.

 

What he said ^^^. Your brake line is bad.

 

I've never seen that happen , but I have no other clue. I believe that is the cause .
My next challenge is to convince my 87 y/o dad that that is the problem .He is convinced since the wheel turns, all is well . Thanks for the input . It may be awhile but will let you know what transpires .

 

have seen the rubber brake line collapse and/or restrict internally several times. happens alot when you have an overheating condition like dragging brakes, stuck caliper, etc..
you saying you could not compress the caliper without removing line is a dead giveaway