When looking for a problem with a double 15 amp circuit breaker, I measure the voltage at the 2 main feeds. One measured 115 volts and the other only 80 volts. Then disconnecting the main feeds from the 100 amp panel they both were 115 volts. My question is if this is caused by a loose neutral wire somewhere in the house circuits? If so what would be the easiest way to find the problem? Or what else could it be?

You got me a bit turned around with measuring the 2P15 A CB, and then disconnecting the main. If you are getting 115 v from one leg to ground, and 80 on the other leg, the problem is not a neutral, as usually if one leg has low voltage the other leg is higher than it should be. The problem can be a bad connection in a breaker or any connection, causing heat and a voltage drop.

Can you be a little clearer about what your are testing and what is being disconnected?

With the main legs disconnected I measure 115 volts to ground at each leg.
When connected to the main 100 amp breaker I measure 115 v at one leg and 80 v at the other leg. When both circuits from the 2p15 a cb are on the voltage drops from 80 to 24 volts on the main leg and every breaker off that main leg. I've tried to replace that breaker and still the same thing happens. When I turn one of breaker swithes off on the 2p15 a cb the voltage to that main to ground goes back up to 80 volts. Also only 80 volts with the 2p15 a cb removed. Thank you.

There is a severe bad connection someplace, may even be ahead of the main breaker, hard to tell because I cannot tell exactly where you are testing the main. There can be a bad connection in the meter or at the splices outside where the overhead utility line connects to your service.

Please call an electrician, as the work on the line side of a main breaker should only be done by a qualified person. The electrician can determine if the problem is is your equipment or the utility, and he should know how to contact the utility and have them respond to check this situation.