2003 GMC yukon XL / Chevy Suburban

[JohnHere]

Be aware that you can't "top-off" the refrigerant by pressures alone and expect to have the correct amount in the system. The only way to get the charge right is to recover what's in there now, evacuate, and weigh-in the refrigerant according to the vehicle's specs using an accurate scale or a Recover Recycle Recharge (RRR) machine.

[tbirdtbird]

Cusser, depending on the size of your ports, they make on-the-fly valve core changing tools that I am sure you might be familiar with.
Google search this, R134a valve core Removal Tool, and be aware that there are several different styles out there for different refrigerant fittings. Even a residential core tool for R22 would have the same 1/4" flare fittings. (since you have R12)
R134a valve core Removal Tool

[Cusser]

Cusser, depending on the size of your ports, they make on-the-fly valve core changing tools that I am sure you might be familiar with.
Google search this, R134a valve core Removal Tool, and be aware that there are several different styles out there for different refrigerant fittings. Even a residential core tool for R22 would have the same 1/4" flare fittings. (since you have R12)
R134a valve core Removal Tool

Thanks, I'm aware that there are special tools to allow changing of valve cores without loss of refrigerant, and don't even know if my local ASE shop has them (he still services R-12 systems too). I'll likely just wait until the system needs service or repair, and take care of the schraders then.

I'm more a "Never trouble trouble until trouble troubles you" person.

A Youtube mechanic Rainman Ray in Florida changes these out every time he does an AC service.