(Also posted by mistake to uk.rec.cars.misc)
Got one of these to-day - the 40 quid jobby with pressure gauge. The AC on the BMW isn't what it was. The gauge showed well low, but the aerosol didn't appear to make any difference. Pressing the button on the can while disconnected didn't exactly give a powerful blast. Shaking it appeared to show it wasn't empty, but there was no seal on it from new - or had the others on sale. Wonder if it had been used then returned? It does give a weight of contents - but not much use if you don't know the weight of the cylinder. Or have I made an error somewhere? The gauge appeared to work - stopping the engine showed a big increase in pressure.