Why do you need to drain front 2 tanks not just the wet tank. Isn't there valves that keep air from going back to wet tank?
Because the check valves may not work and you don't want to be working on a system with pressure in it for safety reasons.
Rich
If the check valves on a 1991 GV are original, there is very good chance they don't work (fouled with trash/corroded/broken spring).
You could take this excellent opportunity to test your front and rear brake tank inlet check valves.
With engine off, normal air (100-120) on the system, drain your wet tank pressure down to zero. Then go look at your dash gauges.
If they are both holding pressure - CONGRATULATIONS - your inlet check valves are still working!
If one or both tanks show loss of pressure, then you need to replace the corresponding check valve.
Part Number Collection (https://www.foreforums.com/index.php?topic=11472.msg225768#msg225768)
Good opportunity to change to a spin-on dryer. I just needed one $9 fitting. R&R is only 5 minutes at the most. See my old posts if interested.
Pierce
Thanks to all who answered. Will check valves and read post about going to spin on dryer
Here are the photos of the installation. You can see the shiny 45 degree fitting in one of the photos. Think I paid a little over $100 for the unit. You need a HD strap wrench to change as the rubber belt models at HF don't work. Still, it's a spin on and easy to R&R in the location we have. Extra cartridges are about $25 delivered. I purchased a clone made in Poland not German OEM. Quality was excellent. spin on air dryer meritor | eBay (https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=spin+on+air+dryer+meritor&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=spin+on+air+dryer+&LH_TitleDesc=0)
Sticking the mount in the powder coating oven is not necessary, any bracket will do. I just filed a bit on mine. Don't know how earlier rear radiator models have their dryer mounted. Getting a "Crow's Foot" ahead of time saves a LOT of work with the big air fittings. Any truck store will have the fittings. They are cheap.
Pierce
I changed my air drier to an adsp also. Mine is a rear radiator coach, I wasn't able to find a cool little mount like that. I had to build mine, it was no big deal. Then the hose from the drier to the wet tank was 6" short. Ended up buying some new aeroquip air line. I really like the thought of spinning the filter off and spinning a new one on. My opinion, well worth it.
I think I salvaged the mount from the old dryer but not sure. Yes, getting stuck on the road with a bad dryer could hang you up for a while, while a rest stop and a strap wrench make it fast. The Snap On tool came from a used tool store for about a buck or so. Good to have one that fits the hydraulic hoses for the later coaches. As I remember, most are 1 inch but I had one 1 1/16" or perhaps it was 1 1/8". Make changing/repairing hoses much easier.
Pierce