I can imagine that this is not exactly going to be useful to everyone BUT
I have a need for a quickly, easily adjustable back support that doesn't involve me moving out of my wheelchair to change my position.

I have at the moment an inflatable cushion down my back so that I can adjust the lumbar support, but it's slow and cumbersome to change the pressure so I decided I needed something that I could change the pressure of as frequently as I wanted and in-situ and so this is what "Engineer and Daughter" came up with...

Step 1: The Plan...

I thought about construction, and this is what I decided I could make/what I needed
As you can see, it's a pretty simple idea and all you need is:

An inflatable cushion of some sort

I'm using one like this

Some tubing

The diameter/length of this depends on your usage, you can get pvc piping on ebay, or screen wash tubing from a car parts supplier, or whatever you happen to have to hand.

A Pump

I wondered about using an electric one powered by the USB socket on my wheelchair but decided there were many more things to go wrong with that than just using a manual hand pump!

A valve

So you can decrease the pressure as well as increase!

Some kind of adhesive

Depending on what tubing and pump and cushion you have you might need some glue to stick it all together. I think.

Pipe diameter adjuster things

I don't know what they're called but we found some from old pumps hiding in the garage. I used one between the cushion and the pipe.

Step 2: Assemble Ingredients

I had a genius plan of buying one of those little pump/valve integrated things for blood pressure monitors and found mine on ebay.
Apparently it's called a Sphygmomanometer and mine was this

Gather your supplies and parts.

I decided that my back support was too tall so glued mine together and taped it (well I did this after but I should have done it at this stage)

Step 3: Connect Piping to Pump and Cushion

The piping we had fitted nicely on my pump and with a little hot water to soften the pipe, we got a good connection

(better than the bulb of the pump anyway - we had to fix that with a cable tie!)

The other end was slightly more complex but luckily we found a pipe adapter which we rammed into the inflation point of the cushion.

I imagine a combination of hot glue and heat shrink could be used here instead of a pipe adapter, although that could be the easiest option!

At this point, you may need to break the valve in the cushions inflating bit if it has one, otherwise you won't be able to deflate the cushion without holding this bit.

Step 4: Test It Out!

Work out how to use the pump

(don't, like me, spend ages trying to pump the darn thing up with the thing unscrewed!)

If all has gone to plan, you should be able to pump the thing up with your hand, then twist the screw bit to deflate as and when you need!

Hope yours works as well as mine does - mine is amazing for me!