Car air conditioning low side port stopped up with something.?

Any vehicle that is older than a 1996 which is when 134a started to be used is ready for the junk yard...NOT time to spend lots of money on the AC system!

There are several options for 12 to 134 conversion fittings. Some retain the original valve and some you have to remove the original before installing the adapter. Maybe what happened here is someone just bought an adapter and tried to install it and its the type that you do need to remove the original valve. The fact that there is not an adapter on the high side kinda points to it not being a proper conversion. If there is crud in there it may be difficult to clean out and you sure don't want to risk pushing it into the system. One little chunk of crud can damage the compressor or plug up the orifice.

The system can be properly discharged from the high side using the proper equipment and then the low side valve can be cleaned.

If the system has never been properly flushed of r12 its not going to last long or work well with 134 mixed in it. The big issue is the oil. The r12 mineral oil does not carry in the 134 and the pag oil typically used with 134 does not carry in the r12. To make it all worse when they mix they turn into a sludge kinda goo. Ester 100 is about as close to a universal oil as there is, it works well with both 12 and 134 and does not react badly with the other oils. If you flush the whole system and replace the compressor and dryer you could use PAG. IF you just do the flush and dryer I would for sure use the ester because you can't get all the old oil out of the compressor.

Remember its illegal to vent either 12 or 134 into the air and before you charge a system it has to be vacuumed down for at least a half hour, both of those items require special tools and equipment that are not cheap. If you are trying to save some money by doing some of it yourself see of you can find a shop that would discharge the system for you. Junkyards may be an option. You then want to ask if they will charge a system you put the parts in. Most shops will but will state that there will be no warranty since they didn't do the work. If you find a shop that will do that you can then do the flush yourself and replace the parts. Don't buy the cheap flush stuff, it does not work. Get the stuff that is like $50 per jug but be sure to follow the directions, its some pretty toxic flammable stuff but it works.

The low side port is the location that you're gonna be using to recharge the ac system,so you don't necessarily need to have a high side r134 port on there unless you're using gauges and hooking both hoses up on your ac tools to pull vacuum,etc.If the system is almost empty of freon or really low,I would discharge what little freon is still left in the system in the proper container,then remove the shrader valve with a valve stem tool,and clean the port out thoroughly.While you're there,you might as well do the same to the high side port too.Use a small pick or screwdriver,rags,or even a small shop vac to clean the ports out good.Then reinstall the valve stems,r134 ports,then recharge as usual