RE:""The purpose of this circuit is to keep an air tank (mounted under the hood, in the engine compartment,) charged at all times that the circuit is activated, with the pump being activated when the pressure drops down to what ever lower end is selected by the user. The tank is used to feed the leaky air shocks line pressure""

Actually I have the 3,5 ton 17-places bus too, with `air-suspension` system, where my bus is able for "ducking" if I release the air out of tank (very useful for older passengers to get in or out) and then it refills by electric compressor at some 2 or 3 minutes. BUT, the refilling takes an energy of some 50 Amps as the smallest. Yes, it leaks somewhere I cannot easy to find where, its true, but it takes a whole night stand to become empty and only few minutes to refill.
However, I use that car only few times a year, for long-range tourism, because normally I go by the smaller car which is far more economic (bus 11 liters and car 4 liters for 100 km). Thus if I left a bus for the night, at morning I shall have a fully pumped suspension system BUT completely empty battery. Somehow I cannot to see any rationale in such `upgrade`. Sorry, if I misunderstood anything correctly.

It could easily be adapted to your purpose. Mine was automated, so that I would have steady consistent handling on the road, without the need to watch a gauge, or stop to check the pressure, manually. Some of the trips I made were in excess of 6 hours, and it was very nice not to have to worry about air pressure, at all. With a heavy load on a trailer, it might run every 45 to 90 minutes, because of leakage. Just one less thing to worry about. I also filled tires on a number of occasions - mine and others, as well as used the high pressure air to clean off chain saws, in the field.

Of course, when the truck was parked, the master power switch was turned off, and when turned back on, would automatically refill to the preset pressure in a couple of minutes. I did leave it on overnight a few times, or should I say, over the weekend. It did, indeed, drain the battery, and force me to jump start the truck. I considered hooking the relay to the power that is governed by the ignition key, but I never bothered, as I just got in the habit of turning off the air system, when exiting the truck.

Keep in mind that my pickup truck did NOT come with any kind of air system, like your bus did for the "ducking". It originally had regular passenger truck shocks, and it wasn't until I switched to air shocks, to increase my effective pay-load / trailing capacity, that I added the air system. Within the first week, I knew I wasn't going to be satisfied with something that required me to get out and refill it all the time. I even started with an air pump that plugged into the cigarette lighter jack, a tire-pressure gauge, and a port for filling in the cab, but that only lasted a couple of weeks, before full-on automation consumed my creative mind energy.

Hope that clarifies any confusion.

 

Well, this is clear, thanks.
My system is made in the way, that if suspension sinks lower than certain micro-switcher, then compressor turns-on automatically, and turns-out as soon the correct height is regained. And if the screw would not be heavily rusted then micro-switcher position is adjustable. Thus the pressure is function of load weight in the car (handy that police cannot see easily if I forward an overweight). However most often pressure is about 8 atmospheres. Really handy when tyre is bit leaking as mine, just screw the rubber pipeline and feel like at petrol station air service. I can drive about day or two, but then air must be added. So I put this wheel to the rear, because price of rubber for one this bus wheel of 225C17+ at my city cost more than for all four wheels of my beloved Passat. Probably at autumn I shall have a voyage via Poland (>1000km one way), and they has enormously cheap prices for rubber.

 

A physical lever to determine when the switch is activated or deactivated makes sense in your application. In fact, the simplicity is wonderful.

In my case, the pressure to the shocks needed to be adjustable, depending on the weight of the trailer on the tongue, and road conditions. That pressure requirement could change several times in a day, as I picked up, or dropped off some loads, or traveled different routes, and types or roads. Therefore, I would adjust it in transit, "by feel", until the load felt good on the back of the truck. By dialing up or down the pressure, I could stiffen, or soften the ride, according to current needs. It could vary by as much as 30 PSI, depending on load carried. Also, the actual height of the frame from the ground was not always the same.

Also, because a pickup truck is a much smaller vehicle than yours, it had a tendency for the rear wheels to bounce, especially where the roads needed repairs. The range of motion from top to bottom, during a bounce, could easily be 6 inches, rendering any physical switch a waste of time, as would be constantly going on and off in short cycles, and wearing out the switch. Larger trucks and buses are built to be more stable, as I understand it.

Thanks for sharing - the concept of using a lever to control the switch may be useful to someone else, in the future. I know I will be thinking about that concept, the next time I have to control a similar device.

 

Ouch, do really the World`s richest nation`s roads has cup-holes too like our`s?? (Thanks a Trump for that )
However, in that Year when mr.putin (those who not deserving the capital letter) occupied the Crimea, I was commanded to work for the month at Crimean observatory (say, give a Europe`s helping hand), so I went to Ukraine with my Passat (and went back just a day before world there became exploded). Never in my life, even when I am cursing our local roads in Balticum with in the mind those glance-roads in the Germany, I had no idea that something SUCH may be named with the proudful label of road at all.
For example, the road from Doneck (now occupied) south to Black sea in whole length of some 300 km was so cup-holy that each few kilometres the heavy trucks and lorries was staying may be weeks left in the middle road because of physical crash of main axle (in the middle to the separate halves). Can You imagine how deep must be the hole let to crush the 15 cm thick high-load designed steel at may be 20 km/h? Then from one end there worked the crane lifting up the lorry after lorry to saddle it on spare axles - the whole year business for heavy rescue teams.
The hole from hole was may be 2 meters afar with 1 meter deep, just hole to hole and narrow pathes where to go, so I drove like through a mad slalom for survival. When at home, I was thrown-out in yearly MOT so I needed to shift all of the steer balls, transverse stabilizers rubbers, wheel hinges balls and rubbers, all about 150 Eur just in the details. And that`s a result from only one 7000 km ride. Other road there was north around the Carpatian mountains from Lviv to Kiiv. All the 700 km length it was just a very deep laundry-board in whole width of road (remember those laundry model from fifties when no machines existed yet). If to go at normal speed, the alive teeth are falling out the mouth, to go more or less normally must drive may be 1 km/h, so the only choice is to `fly very low` at some 150 km/h and then everything is so glide, so shiny glide by feel, but the steer somehow is not very sensitive - You turn right, car goes left etc. Oh poor nation.

 

Remind me not to go on a road trip where you live. Maybe a bicycle, or tank? LOL!!!

The last good road builders were Romans. LOL!!!

I have not seen roads as bad as you describe, but believe me when I tell you that the condition of our roads in the US costs the tax-payers a lot of money on vehicle repairs every year. It has been that way since the 1960's, maybe longer. We do have one of the best interstate highway systems in the world, but the condition of the other roads is often neglected until they are nearly impassible. For instance, there is a small bridge a couple of miles from here that has been out for at least 15 years. Because the road does not get a lot of traffic, the road commission just won't spend the money to replace it. The people that live there, now live on a "dead end" road, where they once lived on a road that passed through. People that used to be "next door neighbors", now have to travel several miles to visit one another.

"Washboard" is a phrase we use almost daily where I live. We have many roads that are "scraped" dirt, and when it rains, they develop a pattern on them that resembles washboards. Some just make the vehicle vibrate. Some can be dangerous at speeds much more than someone on foot can achieve. Some of our asphalt roads also develop that characteristic, usually because the substrate under them was poorly engineered, or implemented. The further the distance between the top of each bump,the deeper the valley between each bump, and the harder the road is to travel. The other expression we use here, is "rattled my teeth".

However, the road conditions my truck's shocks were fine-tuned to accommodate were not washboards, but more subtle dips and changes in the road surface. While it is true that there are some dangerous holes in the roads here in some places, the air system would not be able to compensate for anything that drastic. Instead, it was the more minor dips and changes that would cause side-to-side forces, that would make the steering hard to control, that I was trying to make more manageable. By increasing the effective load capacity of the truck, I still had "shocks", long after normal (stock equipment) hydraulic(?) shocks would have been over-loaded. This gave me better control on the road, when conditions were less than optimal, if I adjusted the pressure to accommodate the load I was currently hauling.

My next car is going to be a hovercraft...

 

You didn't show anything...

The one I used was a ViAir Compressor, 100% duty rating. You can see a (poor) picture of it in one of the last few images, sitting atop the blue air tank.

(I'm not sure I spelled the brand name correctly...)

I use the Central Pneumatic 8 gallon air compressor, It is also decent and 100% heavy-duty air compressor. Its motor produces 4.5 CFM at 90 psi & 5.5 CFM @ 40 psi of compressed air. Its thermal overload protection, oil lubrication of the pump, and high-impact ABS shroud will ensure the reliability and long lifetime of the air compressor. But I need a big model one.

Moderators note : removed commercial link

 

If I remember correctly, Central Pneumatic is Harbour Freight's "In-House" brand. I can honestly say that I have never been satisfied with any electrical equipment that I have purchased in the last two decades with that brand name. In EVERY case, they have failed me, and in some cases, the failures came within days of the initial purchase. I have never owned a single piece of Central Pneumatic equipment that still worked after 50 hours of use,.. most were junk in half that time. I invariably kicked myself for being "cheap", and went and bought a name brand that is known for quality, durability, and longevity - something that I can not say about Central Pnematic. Your mileage may vary, however.

I only mention this, because if anyone is doing an install, and the equipment being installed requires modifications to the receiving unit (in my case, the truck,) you may want to spend a few bucks more for some quality, to avoid having to re-engineer the project, after you thought you were finished.

As far as sizing compressors, my advice is now, and always will be, to buy one size larger capacity than you think you will need, based on pure mathematics. Yes, getting the minimal unit might get the job done, but the simple fact is that the life span of that equipment will be less, than buying one with just a bit more capacity. Besides that, it will perform better (read that, "faster"), than one that is spec'd to barely do the job.

Good luck with your project!