This might be a better question for a plumbing or construction forum, but I figure we may have a plumber here, or someone who can answer my questions.

I want to plumb a sediment filter, a charcoal filter, and a water softener in series. I already have two 10 inch filter bodies, but they have different inlet and outlet port sizes. One has 3/4 inch ports, the other 1/2 inch ports. The incoming water line to the valve under my sink is 1/2 inch copper. Does this difference in size for the two filter bodies pose any problem? Is it OK to go from the 1/2 inch supply at the value to a 3/4 inch filter to a 1/2 inch filter, and then to 3/8th (roughly) for the water softener, and then down to the final espresso machine line size?

I'll give it a go. I am not a plumber but have had allot of experience plumbing.
It all depends upon what your final flow rate needs to be. If you are only feeding an espresso machine or filtering water for one water faucet it does not matter too much what size the fittings are. Typically the fitting size is determined by the manufacture for the max. volume of water that the item is capable of passing effectively. If you use this item for a lower flow rate then usually they work just fine. for a single use faucet I have seen the use of 1/4" fittings and they have worked just fine (depending upon you supply pressure of course).

As long as you have pretty good pressure and eough volume of water coming from your water supply, the use us filters/softeners etc. with fitting sizes equal or greater than 3/8" will most likely be OK.
The worst case scenario is that you directly plumb an espresso machine and after you connect all of your water filters etc. you do not have enough water volume passing through your system to keep up with the demand of the equipment.

BillC

Thank you Bill. It's been a few years since I tested my water pressure, but it was around 55 psi as I recall. If I open the faucet all the way, I get far greater flow at my kitchen sink than I usually want. So it sounds like I'll probably be OK. I'm planning on putting a pressure regulator and gauge after the water softener to reduce the pressure to the machine. 30 psi seems be what many people advise.