Two years ago our known and trusted Shurflo 2088 fresh water pump failed and although I had a spare (in theory), through laziness and procrastination I hadn't got round to rebuilding it (the new parts were all there - somewhere!) - so in order to keep the taps flowing I popped over to Midland Chandlers to buy a new one. The 2088 had been 'replaced' by the 'all new' Aqua King 3.0 (3900 series) so I bought one of these and fitted it. It wasn't a direct replacement because although the 1/2 inch water fittings were the same size and in the same place, the four rubber bushed mounting holes in the plastic baseplate were at a different pitch - this was easily remedied by taking the metal baseplate off the failed 2088 unit - the holes and screws attaching the baseplate to the motor body being identical - a point worth remembering.
Now two years on, the 'new' Shurflow Aqua King 3.0 has suffered a pressure switch failure - fortunately, in the intervening months, I had got round to rebuilding my spare pump so a quick changeover was possible. The old 2088 series pumps had a whole range of readily available (and affordable) spare parts - and as the most vulnerable part seems to be the pressure switch I had sourced a useful supply of these.
Unfortunately, the pressure switch on the 3900 series is a completely sealed non serviceable unit. This, apparently, is progress!
A replacement sealed unit is available from my favourite chandlers for £29.99 which is, incidentally, the cheapest price that I have found anywhere for this part. Most suppliers are asking around £35 which is remarkable considering that a complete 3900 series pump can be obtained for less than £70.
In conclusion, I reckon that the only 'improvement' that has been made with the 3900 series has been for Shurflo's profits - derived from more frequent replacements. At least the motors are recyclable!