ongoing radiator cap leak, ??? long read

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I can just deal with one of these ata  time.

As for the air compressor, never replace those without first finding out where the system is leaking.  Most likely you have one or both shocks leaking, it's only a $75 or so problem, you need a pair of Monroe MA-822 or similar (see RockAuto.com), I have KYBs on and I like them very much.   (see my Sig)  That said you can't go wrong with the Monroes.  

First just look at the shocks.  If the shocks are rusty you probably have found the problem; confirm it with a leak test (which you are going to do anyway). My guess is they are leaking, you say you hear air hissing behind the tires, well that's where the shocks are.

You do the leak test by getting an empty 32 oz spray bottle (like an empty Windex, or just go to the dollar store and buy a $1 window spray cleaner in a bottle).  Fill bottle about 2/3 to 3/4 full of plain tap water, then add about a teaspoon or so of liquid dish detergent.  Then you spray down the outside of both shocks until wet, then start the car in Park or just turn the ignition on if your battery's in top shape) and let the air compressor start.  The shocks have to be dripping wet with the bubble solution, you can keep spraying to keep them wet while you look.  When you are spraying the solution on the spray bottle may make bubbles form, that's not important because if one of the shocks is leaking you will see a stream of bubbles coming from the leak.  Once you have seen this once it will be obvious forever. You can also use this approach to spot the leak points in leaking residential natural gas lines - try not to make too big of a mess but any mess you make can be cleaned up versus an explosion; explosions are much more problematic.

As for changing the shocks - you will need new speednuts for the bottoms, they don't come with the shocks and they are often frozen in place. They are also cheap. hardware store or auto parts HELP rack items. Take a look at yours then buy 4 of them and bolts to fit.

If your dexterity is good you won't even have to jack the car up to change the shocks. You *should* jack it up, it's easier. Be sure to set the brake and chock the wheels. Then basically you unclip the air lines (when the car is parked they are vented to about ambient pressure by the computer so no drama like with a Lincoln), then unbolt the old ones and install the new ones. You *could* install new shock mounts but frankly the rears don't take that much of a beating so if the rubber (on the OE and better replacement mounts it's not really rubber, it's a specific elastomer and I forget the spec number) looks good you can get away with re-using them. As for me I just replace them but in 330,000 miles I've never seen/had them clapped out at replacement time. Anyway, first LOOSEN the mount bolts at top and bottom then remove the tops, remove the bottoms (might be easiest to just break the speednuts by tightening them to failure rather than trying to loosen them if they are rusted in place). Inspect the old mounts and replace if required - or if you bought new mounts just put them on the new shocks. You will be able to turn the shocks so the air nipple is pointed the right way while you are installing them. Installation is generally the reverse of removal. Test for leaks then take the car off the jacks. I have heard of but never myself seen leaking nylon air hoses or leaking air connectors on a Riv suspension, by the way.

I am going to re-read your post vis a vis the other problems, you piled them on pretty thick.

As for the radiator leak. You probably don't have a leak in the coolant loop that goes to the heater in the passenger compartment but you might want to check. If you smell antifreeze while the heater is running, you probably need to replace the heater core. By the way, if it smells musty in teh car when the heater is running it might be a core leak or it might just be dirt - make about a gallon of disinfectant solution using Lysol liquid, odo-ban, Dettol or what have you and pour that evenly into the cowl vents (the vents just below the windshield wipers at the back of the hood)

Also, did the mechanic replace the hoses to the heater under the hood? They are a royal pain to replace but should also be at least checked. They are not likely to be leaking; if replaced they may be leaking at the clamps. I replaced mine at about 150-160K miles with Gates hoses, and don't think I'll need to do it again because of updates to the construction and composition of standard auto coolant hoses including these.

Next, those hose clamps. The original spring clamps are mad tight and generally don't have the problem you saw with leakage at the band clamps. The headache is that you should use a special too to get the spring clamps on and off. Some mechanics don't have that tool, it's about $40 and you don't use it much. The issue with the band clamps is not the clamps themselves, just that the some of the coolant hoses are hard to get to for cinching a band clamp. No surprise you had one loose, you probably should check them all yourself. You'll need a screwdriver or nut driver depending on the clamp. Just go look.

As for the radiator leak, instead of an OE cap you may want to get a 16-lb Stant from an autoparts store or even WalMart if they have them in stock. I went through 2 OE caps before switching to a Stant. I have an OE cap for parking the car with hood up on cruise nights...

Oh by the way, DON'T let the coolant run low especially on this supercharged engine. Keeping the coolant up is MUCH less expensive than repairing this engine after it is allowed to overheat.

As for radiators, the aftermarket Spectra Premium has a bigger oil cooler (6 plates) which should help your transmission in the long run. I replaced mine with a Valeo (at ~150K) and it's still running happily. I bought the Valeo because it had extra rows. I do not know where to get a Valeo now, though.

You should top off the coolant - if you are still running Dexcool (it's orange or maybe pink), get some Dexcool (again, walmart) and refill the cooling system. (if not dexcool, then the 100,000 mile green "mix with any" stuff) Before you refill, open the radiator cap and dump in some dye (get the dye at a car parts store like autozone or advance). Finish filling at the radiator. Don't bother putting dye in the overflow bottle, just put in the antifreeze/water mix, you want to be able to confirm that the coolant expands into the overflow when the car warms up and goes back into radiator when it cools and anyways leaks there are easy to see. What you have to do is top off both the radiator and the overflow bottle. The service manual has specific instructions for this, hopefully someone will post them (don't have mine ATM). By the way, you may want to use distilled or de-ionized water to mix your antifreeze if you're not buying pre-mix. I use condensate from a dehumidifier.

After running the car for a day or 3 with the dye in, use an ultraviolet light to find evidence of the leak. In a dark area, or at night, open the hood and shine the black light on the engine compartment. You should see residue glowing in the vicinity of the leaking. NOTE: This may not work. On my Riv I had a hairline crack in a radiator tank that sprayed coolant perfectly onto the ground. There was no evidence of dye on anything in the engine; I found out almost by accident while looking under the hood with the engine warm and running. The mechanic at the dealer had put dye in the system - twice - over the course of a couple months while we were trying to find this stupid leak. I actually forget why I was looking under the hood with engine running -- I was troubleshooting something else -- and I noticed the coolant wizzing in a threadlike stream from the driver's side radiator tank onto the pavement. You could not see the leak with engine off and there was no residue anywhere. I am not sure what's going on in your case -- is it a problem in the overflow recovery system, maybe a different clamp missing or loose, or cracked overflow bottle or some silly thing; or the radiator was damaged on install maybe (rarely if ever happens) or come to think of it there is one more thing.

When you work on a Riv cooling system, or any other GM 3800 engine from the late '90s (and the Northstars too), you most likely need to put stop-leak tablets in the coolant. If you don't the engine will usually seep coolant due to myriad issues. The tabs are available from autoparts stores, many truck stops, walmart and many GM car dealers have them in their parts departments. GM call them 'cooling system sealant tabs.' They are sold elsewhere as 'Bars Leaks radiator stop leak tablets.' I think they are made from ground up ginger and nut shells or some darn thing.. but they work perfect for this application. My experience -- follow the label directions. IF you have to top off again and there's no apparent leaks consider adding one more (that's why there's 6 in the blister pack when they say you need 2-3, duh) but DON'T put in too many or you'll clog the heater core. On my car, after a coolant change (I've done 3) I need to add an extra tab the first time I top off after the coolant change. You really should read the GM explainer on this; you can see it on the Cadillac forum at this LINK. GM quit using the tabs routinely after making some manufacturing changes, BUT still uses them from time to time. People were complaining about the visible residue in the coolant overflow from the tabs.

You have a Riv with a bunch of miles on it -- you might/should get a copy of the factory service manual set, they come up on eBay from time to time.