I installed a Fluidmaster 400A almost 10 years ago. There is excellent water flow into the overflow pipe, but the tank is taking longer and longer to fill after flushing--now almost 4 minutes. Much water is thus being wasted going down the overflow pipe. Everything else is working perfectly. Do you recommend removing the valve cap assembly to try to flush out debris from the fill valve (I have instructions for that), or should I just buy a new 400A and replace the old unit? (I always have problems with leaks when I do replacements.)
Cleaning out the valve should help. On the other hand, a new 400A fill valve is about $7.50 and it will take you under 5 minutes to replace your existing one (water off, slide the retainer ring up, pull the old valve, throw it away, put the new valve down at the same height, click the retainer ring back down, re-attach tube to overflow, water on).
I'm pretty sure any pro would just replace the valve to save time.
Yes pull the cap and flush the valve. I have done this many times. Turn the water off flush to dump the tank and bleed the line. Remove the cap. Place a cup or bowl over the top and while holding the cup in place turn the water on. You might get a large amount of fine sand, rust pebbles, even pieces of pipe sealant all things I have found in the past when doing this. Turn water off replace cap, possibly clean debris out of tank. Turn water back on and see if that cleared the blockage. There have been times where I got loads of stuff out and it did not help but most of the time it has fixed the problem. With low flow and leaking.
There's a couple of videos working on this subject
How to repair slow filling toilet This guy had a 4 minute fill time, he removed the float and removed some kind of plastic restrictor insert that seemed to be the culprit. It might be some kind of pressure regulating device, he didn't seem to know its role or function, he just removed it.
TOILET FILLS TOO SLOW!!...QUICK AND EASY FIX! This guy tackles the fill master specifically by disengaging the fill valve top which is designed to be taken apart and flushed. He flushes the fill valve and the side spout thing and demonstrates a quick refill. Now I did this before on my unit, it will refill quick the first time after you shut off the water but the second time after a flush it will not refill quick.
How to Flush Fluidmaster Fill Valve This woman does a great step by step overview while explaining each step and even symptoms to watch out for. She does the same operation as the second video above. She gives good attention to the replacement main seals which could be an alternate way to resolve the problem.
Replacement Toilet Fill Valve Seal for 400A Fill Valves this is the replacement part that is known to go bad. I'd like to better understand the mechanics of exactly how it fails.