Sounds like two completely separate issues.
1 - Sounds certain you have a fairly large leak, but are you using a proper gauge set to check the pressures? Are you sure it's empty?
Have you tried using A/C tracer dye? It's not expensive and should be pretty obvious where the leak is coming from. Alternatively you can get an electronic sniffer that will detect leaking refrigerant, such as this one:
https://www.amazon.com/dp/B083XTT26M/?tag=serious-20
At this point I wouldn't' take anything apart until you narrow it down. Old systems don't like to be disturbed and you might just be making more work for yourself.
2 - For the lack of heat you can narrow things down.
- When you change the heat setting, do you hear the door actuator moving behind the glovebox? If the actuator doesn't move, the actuator could be bad or the control head could be bad.
- Does the truck get up to the correct operating temperature? If not, you are looking at a thermostat problem.
- Do the heater hoses going into the firewall get hot? If these hoses aren't hot, then nothing you do inside the cabin will fix it. Your water control valve might be bad, or somehow the hot coolant isn't making it into your heater core.
- Do you have enough coolant? Low coolant can cause a lack of heat.
It's possible your heater core is clogged, but I would do the above first. You can check this fairly easily by removing the hoses going into your firewall and blowing some compressed air or water through it.
Also - I would do the above first - but common failure is the blend door inside the airbox. You can replace the door without removing the airbox and its not expensive, but it is an annoying job. Basically you cut an access hole in the airbox and remove/replace the broken door. The airbox is just plastic and you can use something like a "hot knife" to cut through it without much trouble. Basically a soldering iron with a blade attached to it.
