I'm adding a air up,air down tag axle to my 1998 f800.As it is the truck doesn't seem to hold much air.I think when i hook up the tag axle,the truck will run out of air when putting the axle up,or down.I want to add another tank but i don't know how big of a tank i'll need.,Is a 5 gallon enough?

 

You don't need more air supply, because its a one time thing. What I mean is you switch your valve and the axle will rise, build to set pressure than not require more air until you lower it again, then it's the same when you lower it (unless you have leaks of course). It doesn't matter if it takes a few minutes to do so. The air compressor will automatically load and compensate for the air usage. You can add an additional tank if you like but it's not required for the air lift. BUT if you have air brakes on this axle then you may need to add to your reserve because now your consuming air as you drive and you require enough storage to compensate as per DOT requirements. I don't recall the exact specs anymore but a two gallon tank is more than enough (five gallons are probably cheaper though because they are much more common, used on trailers)

 

You should definitely add an air tank when installing a drop axle.

Just the extra set of brakes would be enough reason. Like you said lifting and droping the axle will drain the tanks very quickly, especially if you have the drop hooked to the reverse switch, which it should be unless you're the only driver.

5 gallons will probably be ok. But 10 would probably be a better bet.

I wonder if anyone knows what the DOT says a set of brakes should have in reserve? I dont remember ever hearing the rule?

 

It's been awhile since I was in the business of modifying trucks but in our jurisdiction they are starting to crack down on modified vehicles. Inspection mechanics are supposed to look at the original equipment stickers on the door and companies that modify vehicles are supposed to have everything engineered, do weight and balances then install modification stickers to the door frames with the OEM specs. This includes frame, brake, axle, fuel system, exhaust system, and other modifications.
If this is a commercial vehicle that requires periodic certifications, you could be opening a whole can of worms.

rummer; there is a whole lot of stuff you (and I) haven't heard of, in away it's common sense, it's simple, you need enough air to apply your brakes a certain number of times, it's in the interest of public safety. It's all there, if you want to get a taste, go to a shop that does CVIP inspections and ask them to look at their inspection manuals, the shops book is an inch thick, then each inspection mechanic must have a book that's an inch thick, these books cover everything from inspection procedures to the type of lenses on the marker lights to the way you fill out and validate the forms to DOT approved stickers on the doors.

There's allot more to it than many people realize.
Sorry 6t56, don't mean to discourage your project, just stating the facts.

 

As far as tag axles My experiances are they dont use that much, as said
above. I use it like this, go to plant get say 15T asphalt put axle down
go to job, axle up. No air is used unless you gotta leak. Plus side, you
got more brakes. My problem, talk about using air, is my F8s got air
piston for plow. Pushing back street corners-back and forth I can barely
stay at 60 and I have three tanks. And man we got some snow, my
poor compressor. Took me 60yrs to find out there is no best of two
worlds like my F900 has electric hyd. plow so that runs all night with
a dead battery.

 

That's something else again, now you need a larger compressor, it doesn't matter how much air storage you have, if you can't recover your pressure (DOT reqires max 3min 50psi to 90psi at 1200rpm but that is slow) then your gonna spend all your time building air pressure in the case of the plow. The same applies to your altinator and battery, if you use more energy than you produce it has to come from somewhere .