Put ice cubes in the disposal (about 1/2 way), run the disposal, flush out with cold water. Next put 1/2 a lemon and grind it up.
If you can see water in the trap , then the trap is holding water and you know it's working. Even if the trap is undersized (1 1/2"; instead of 2";) it would work. So the problem is probably a leaking drain pipe, the shower drain itself (the part that is connected to the shower stall) or it's leaking where the two connect. Can't fix the pipe or the drain itself without pulling the shower out. But, if you can see a rubber or lead ring around the pipe as it sticks up into the shower drain - that can be removed and a new one put in. This is a pretty common practice in concrete shower installs.
Up until 10/15 years ago we put in anti hammer tubes. The *powers that be* found that the tubes got water logged over time. So - it's no longer code. In commercial installs, spring loaded devices are used especially at the end of long runs or at the end of a series of fixtures like urinals. To replace the air in the anti hammers, drain down the water in the whole house with the faucets turned on. The idea is that when you turn the water back on it will compress the air at the highest point at the end of each pipe. That's what the *powers that be* realized that plumbers were not plumbing for - and home owners would not do.
There is no way to clean rust out of old galvanized pipes. Most DWM s have a screen where the water connects to the machine. You access it through the lower front panel. Another solution would be to put a filter on just the hot water pipe to the DWM. If you replace the pipe - use copper - and connect to the old galvanized with a dialectic union.
Find out what the freeze depth is in your area and bury the pipes below that level. Here (Seattle area) it is about 2 feet. Use schedule 40 PVC for cold water and CVPC for hot (if you are running that out also).
If the run from the house to the street is short (under 60 feet) I'd use *Type L * soft copper. It is less likely to break and it has no fittings in the ground except at each end of the pipe. I'd also put pipe (foam) insulation) around the copper run. For longer runs my next choice would be schedule 40 PVC pipe. Not a bad choice at all. I would not use flexible plastic - that is black *poly* pipe. It comes in a roll. Way to soft and the metal clamps and hard plastic (or metal) connectors will break over time. I repair them weekly.
My Black Poly Pipe Pet Peeve My experience with black poly pipe (comes in a roll) is that it leaks. It is too soft. Hard surfaces (rocks) rub holes in it and where ever there is a transition to another type of pipe or an elbow it will leak there - because of the hard adapter, radiator clamps and the soft pipe. Schedule 40 PVC or copper is the best way to go with water mains.
If you lived alone, only using one plumbing fixture at a time - correct pipe size wouldn't be a big issue. However when you are in the shower and someone flushes the toilet - it is a big deal. The basic rule is *two fixtures on a 1/2"; pipe*. You need min. 3/4"; incoming cold pipe for a one bath house. Just running 3/4"; to each fixture in the house won't hurt, but there will be no real gain. To size a water distribution system, get a copy of your state code book. It will spell it out in terms of beginning pressure, the furthest fixture from the meter and the number of fixtures in the house. Each fixture is worth *so many units* and you are allowed *so many units* for each size pipe as you get further from the meter.
IMHO copper rules in most situations (unless you have low pH or aggressive water) ! Over time, the plastic can sometimes *sag* and possibly get brittle. If it needs to be repaired or altered in any way, the pipe will have to be glued and you will have no water 'til it dries. Copper-you can solder, turn it on, test it and know that all is well. Any plastic to metal connection is weak, such as where the HWT connection is made. Mice and rats love many plastics. They chew on it to keep their teeth from growing through their lower jaw.
It's my understanding that having a disposal is like having another person adding to the load on the septic system. So... are you currently under utilizing your system, over using or about right? Dishwasher on a septic system? I don't think really matters. If I had a dishwasher - I'd run it into a disposal even if I did not use the disposal for anything else.
Garbage Disposal problems can be plumbing, electrical or appliance. If the disposal doesn't work at all - no *hum* or any sound, then push the red (reset) button on the bottom of the disposal. If that doesn't work, check the breaker in the electrical panel. If the unit *hums* but doesn't turn then you can try unsticking it.. Some units come with a wrench that you can use to turn the cutter flywheel from the bottom. ( An allen wrench will work). Or, use a *plumber's friend* , broom handle- something with a handle, and stick it in the disposal and try to turn the cutting wheel around. In effect - unstick it. If the unit doesn't respond to the above, it's time for an appliance repair man or replace the unit.