Unable to manually fill rear air suspension with air compressor

I have not heard of filling the air springs manually on an original system. You might try the Rock Auto web site when you need parts for this system. They list a lot of the original parts and even list the compressor. The only advantage to this system is it will self-adjust for variable loading, meaning for a different number of heavy items in the trunk, (or luggage!). If you fashion a way to fill the air springs manually, you will get the ride height back but not the self-adjusting feature. Look for a store like Toys For Trucks or an automotive speed shop that sells air shocks. Ask if they have just the fill valve that can be purchased separately. That will have a standard tire valve and two ports that accept the small plastic hoses, one for each air shock.

"The only advantage to this system is it will self-adjust for variable loading, meaning for a different number of heavy items in the trunk"

9 times out of 10 the air shocks are leaking causing the issue here is how you replace them. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.