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This has me stumped. In one of six zones I have low pressure. it is low whether I bleed it by turning the knob or if the electric signal opens it up.
I switched out the diaphram first in an effort to fix it. No luck. Then I swapped out the entire top of the unit, including the diaphram, diaphram spring, solenoid, screws, etc... No luck. Then I looked for signs of a leak. Nope. For good measure I used pins and brushed to clean out all the little holes.
Then I tested it by unscrewing the solenoid a bit while the pressure was on - and the system ran at full power, but leaked around the solenoid. When I screwed it back in fully, it stayed at full power. Then I turned the system off and back on - and it went back to low pressure. I unscrewed the screws holding the diaphram down a half turn each - and bingo, full pressure, but leaking at the edge around the diaphram. When I tightened it back down to end the leak, the pressure stayed high... until I shut it off and turned it back on again. Then it went back to weak.
It looks to me like the diaphram is not rising all the way up unless I create a leak in the valve. Then it goes up just fine. However, even after I replace every moving piece, the same problem persists, and the diaphram doesn't go all the way up.
Guesses as to the problem? A defective base that won't let the diaphram move correctly? It is the only piece I have not swapped out... Should I bite the bullet and replace the entire assembly?

Quoted from "mrfixit"

Orbit 3/4".

I took the replacement parts from a valve that was working well. The parts are all less than a year old. I'm pretty sure I've cleaned everything and can't see anything pinched.

I'm still confounded. I've thought I had it solved a bunch of times. At first I thought it was the pin at the top of the top part - it is on a separate piece and when water gets behind it, it can get pushed down. However, that was not it. Even after supergluing the pin piece to the top part at the highest level, the water pressure still drops if the unit sits for 12 or so hours unused and then is turned on. It is clear to me that the diaphram is not going fully up. Then, if I unscrew the top just a bit, the system goes to full power and stays there, and even comes backon at full power if I turn the unit off for a few minutes and then turn it back on. I've removed the spring above the diaphram, weakened a spring, snipped a tiny bit off the pin, flexed the diaphram to give it more give, etc... Nothing makes a long term difference.
I've replaced every piece except the base. I've taken off and cleaned every part of the system multiple times using every technique I can find. I think I just have to face facts - the valve is defectively designed. The one that is the worst problem (because the sprinklers are spaced most widely on it) is going to be replaced soon to see if a new valve makes a difference.