FYI 2000 Blazer

Hello all
My brother called me today and said his 2000 Blazer heater is acting funny. While driving he has good heat but at a stop light the air gets cold. He also said that he did change the thermostat and that did not help.
So I asked him how is the rad cap and his response was its good. I then told him to take it out for a 20 min ride and the open the hood and see if any of the rad hoses are hard( psi) So he did it and called back that there was no psi in the hoses. Well replace the cap. He did that and still the same so I asked him if he had any type of water running sound in the dash and he said yea all summer it was like that but he thought it was the ac system. So I told him that there was air traped in the heater core and told him how to purge it and when he called back he said there must have been 3 min of air purging out of the core and when the air stoped and he got all coolant he had tons of heat.
I told him that is a common problem al just about all S-10 , Blazers have or will have it. The heater core is the lowest point in the cooling system and air gets traped in there all the time.
Fireplug

Help a fool out....I just changed the waterpump and thermostat. How do you purge the system of air?

Help a fool out....I just changed the waterpump and thermostat. How do you purge the system of air?

Help out another fool, as I'd like to know as well. My 2000 Blazer has been doing the exact same thing.

Thanks in advance.

Usually needs the rad cap replaced seeing that is usually the source of the air getting into the system in the first place.