This has been going on for some time now and since accessing my pressure switch and bladder tank requires a trip into the crawlspace I have neglected checking things out until today. Basically whenever there is water running (sink, shower, toilet, etc) the pump cycles constantly. On for 2 seconds then off for a few and repeat. So when I go down there this evening I was shocked to see the pressure gauge showing 100psi. I rapped on the gauge and it didn't go down. It actually went up and past the 100psi mark. That's as high as the gauge goes so if I were to guess it was showing 110psi. So I signaled my 4 year old helper/son to flush the toilet. When he did the pressure immediately dropped to 35 and the pump turned on. It went to about 55 and turned off. This repeated for as long as it took to fill the bowl. I watched it for quite awhile after it stopped and the pressure stayed constant at 55ish. Didn't drop or go up. so I'm not sure how that gauge was reading so high when I first checked it. Now to provide a bit more info...Lived in this house for 7 years. I was told that the bladder tank was newer at that time. It's a Wel-X-Tol model and is mounted horizontally between the floor joists. The tank does have a base where it appears that it could also stand up. I noticed that there was a small drip coming from that base as well and some rusty looking water with bubbles. It wasn't actively bubbling just had bubbles sitting there. I'm thinking the tank has gone bad? Also, can these tanks be mounted horizontally if not designed specifically to do so? Can't wait for this bill... Just when we were about to start Christmas shopping. UGH. Am I correct here? It's a bad tank?

Best case scenario: your tank is waterlogged and simply needs to be charged with air to restore the proper air volume.

worst case scenario: the tank is bad (diaphragm) and needs replacing.

probable scenario: the tank is bad internally and externally (rust/blisters).

I have found in my experience that tanks can be mounted horizontally but that they tend not to last as long. Is there any way you could install one vertically?

thermal expansion would account for the high pressure, and as M&M says your tank is either waterlogged, or bad. In either case you would probably be better off to replace the tank as it is not in a very accessible location. Is there any way that you could relocate the tank if it can't be mounted vertically? If you could get it into a more climate controlled environment and a more accessible location you would be better off.

Two things.
First, on the top of the water tank there is what looks like a tire fill stem. Use the end of a knife or a screwdriver to push The center of that stem and see if water comes out. if water comes out then the tank is bad and needs to be replaced. if water does not come out it could simply be that that valve has leaked and the pressure has gone from that bladder and will require you to add about 32 PSI to that bladder if you were not sure what differential pressure you need you can go online that any bladder tank or expansion tank manufacturer and find that calculation. "Usually 15 psi less than your incoming water pressure"

Also take time to learn the difference between a bladder tank and an expansion tank they are two completely different devices if you are on a well you will want to have a bladder tank. if you are on city and simply are on a long run or fluctuates due to use of water in the neighborhood you will want expansion tanks and I say tanks because you will want 2 of them. You want to install one near the main as it comes into the house and the other at the opposite end of the plumbing system either in the attic or under a sink (connected to the 1/2 supply with a tee before the stop) in the farthest bathroom this will prevent water hammer and maintain a constant flow when you are using the portable water system.

Thank you for the informative responses. Relocating the tank might be a challenge. The crawlspace, while not technically climate controlled, does stay a pretty consistent temperature. The home is built on a geothermal foundation so it stays in the 50's year around. I'll check the stem to see what comes out when I depress it. If bad I think that I'll insist that a horizontal model replace what is there now. Wel-x-trol seems to be the quality option. Any others that are worth looking into?

Well X Trol Professional, 7 yr warranty- best out there.