The thing I don't like about the electric timers is they will run 24/7 unless you unplug them. Also I've seen more than one where crud from inside the tank will get clogged in the auto-drain and it keeps it open. I can't tell you how many times I've come to work only to hear the neighbor's compressor hissing out air for who knows how many hours because their auto-drain got stuck.
I like the pneumatic style better, it cycles when the compressor runs, and most have a shearing type action that have less of a chance of getting stuck open.
I've read of people taking the harbor freight model and replacing the o-rings inside, and also using copper tubing instead of the plastic that is included. The main fixture itself is brass I believe, so kind of hard to go wrong with that.
Personally, I just like the regular old manual style the best. The amount of water that gets trapped in mine is less than what is in the pipe so the tank should always be stay pretty dry.
I totally agree with this:
electric timers:run 24/7 unless you unplug them. That is the reason I made it a point to indicate that this is a
PNEUMATIC triggered dump. It ties into the line that goes to the cut off switch, the same pressure drop or rise that signals your compressor to start and stop activates the dump. Btw this valve operates in a rage of up too 300 psi, it will handle any size compressor found in our garages.
The biggest problem with the harbor freight model is the working pressure, they are only good for 120 psi max. I had one in the past, read as removed and replaced with this MM. I got it to work, plumped in a regulator in front of the harbor freight valve. Reduced the pressure to 100psi, pressure for my compressor is 175. Even at the reduced pressure setting to the valve only it dumped/hissed the whole time the compressor was recovering. The MM hits a lick dumps at start up and again when the compressor recovers, tops off.
Notice that the valve is on the side of the tank and higher than the bottom of the same. It works there because the pressure in the tank will keeps the water forced into the line, reservoir and out of the tank. That should be common sense but point it out for clarity.