Slow pressure increase

[tbris]

Each of my six zones works at approximately 75% pressure for the first 5-8 minutes, then eventually ramps up to the usual area coverage. Never did this before. I read an earlier post about low pressure in one zone and the recommended fix was either a new solenoid or diaphragm. I'm wondering if that could be the issue for me as well.
I have a Watermaster 57223 auto valve model. 3/4". Due to placement, I'd hate to have to replace the entire sprinkler valve.

[upper]

City water or your well?....Upper

[tbris]

City water. System is at least 5 years old. I have been using it problem free for 3 years.

[upper]

So if you took 2 or 3 garden hoses and turned them on,the would not fluctuate greatly? Second all zones do this equally?Thirdly when a zone comes on and is lacking,go and manuallly turn the solin. lefty lucy and see if this helps...Upper

[tbris]

The hoses would not fluctuate greatly...pressure is solid at my house.
All zones do this equally. There are 6 zones total.
I haven't fiddled with the solenoids yet. It seems odd that all six would need a tweaking.
I tried turning lefty lucy and saw no noticeable benefit.

Thanks for taking the time to help.

[BRD]

Seems to me that if you're having the problem in each of the 6 zones that the problem is between your water source and the valve manifold. Very doubtful that the problem is in all the individual zone valves. Do you by chance have a master valve in the system? I would track your plumbing back to your water source tie in point. What kind of backflow device do you have? Is there a pressure regulator in the system? These are some of the things you need to check. Especially since it is a city water system, it should be easier to track down.