My 5303 loses air from the rear bags on the curb side when sitting for a day or two. The rubber bushing in the level control valve linkage was completely gone, just had a bolt with no bushing at all it it. I think this, combined with a faulty check valve, wasn't helping.
The rear valves look fairly new. I need to check and see if they even have a check valve on the inlet. If they don't, any air leak at all will be magnified as soon as the valve calls for air, as it will just leak bag pressure back into the suspension tank and fall.
I found these standard adjustable links to replace them with. I may leave them an inch longer, I need all the rear clearance I can get.
(https://lh3.googleusercontent.com/FLvVNVsHDVmweSnbIBUkYxIcVcwHeNJA4bwANszYj1peGQXjaDnAz8z6lNh0ufEs48fjFA-ibLuju8dMjv-RitL4QRfkzdxqXaWD_lBIDSFh574HE3e4ChPS07np33oTpUDiwhvIg24)
I'd bypass the valve temporarily and rig up an adapter with a Schrader or valve stem type end that you can add air with a tire inflater on an air hose. No compressed air source? Put one on your dry tank, which will come in handy in future. Inflate bag/s to say 60 lbs and narrow down leak to single bag by isolating to two on one side or other, then one. Being that old, I would expect more than the leveling valve is the culprit. I did this several years ago on a 4905, which was a leaking rear air bag. It cost very little in time and money.
Here is an example, but you can get these at most auto parts store, esp. NAPA. PS, put a little spit on end of Schrader to make sure it isn't leaking after inflation.
https://www.grainger.com/product/1KUH4?cm_mmc=PPC:+Google+PLA&s_kwcid=AL (https://www.grainger.com/product/1KUH4?cm_mmc=PPC:+Google+PLA&s_kwcid=AL)!2966!3!50916756597!!!!82166066877!&ef_id=Vr-02wAAAbMaqfEA:20171020133804:s&kwid=productads-adid^50916756597-device^t-plaid^82166066877-sku^1KUH4-adType^PLA
The bags are fairly new, along with the valves. I've got a male quick connect on the drain of my aux tank, which will fill the suspension tank once it reaches 60psi and activates the PP valve between them, so I can get air into the system pretty easily. Next time I have it on the ramps, I'll get under there with the spray bottle of soapy water and find where it's leaking. Drivers side stays up, along with the front, so it's got to be the valve, lines, or bags on the curb side.