I have two 304s. I used to have problems with both of them now and again.
Since I have started doing the following I have not had any problems.

- keep a towel handy because it took me several
times to figure out exactly how high to fill the canister
and the canister often leaks until the pump is running
and the pressure builds up a little.
- use the aquastop to "lock" the water into the intake
and output tubes.
- Remove the aquastop and tubes from the top
- remove the top
- rinse all of the media, etc.
- put everything back in the canister
- put the top back on.
- prefill the canister all the way to the top where the
aquastop goes with tank water using a siphon tube
- insert the aquastop and lock it in place with the lower
lever
- open the upper lever to allow the water to flow from
the tubes
- pump the manual priming pump until water flows from
the output nozzle with each pump
- power on the pump

At this point it will usually fire up without any trouble. There will be a
lot of air because the media was uncovered while rinsing. This will usually
clear up on its own. However, I am usually impatient and will tap the side
of the canister and shake it back and forth to get the air to release more
quickly from the media.

Since the 404 is larger than the 304s I have you will have more media and
therefore you will have more trapped air. This may give you more trouble.
You can shake the canister after filling and before attaching the aquastop
to get as much of the air released as possible. You can also top off again
with water after you think you have gotten all of the air released.

Hope that helps.

"Bill Stock" wrote in message
news

"Lobsang Mack rogers com)" (is.guy atdot wrote in message
...
I have a Fluval 404 and it is to the point where I am afraid to touch it.
Everytime I break it down for maintenance it is a bugger to restart.....My
103 was a dream...never a problem.

I replaced the primer unti and when I try to prime it it still sounds
like it is sucking air. The impeller is spinning but no water is moving.
ANy ideas?

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I've got two 304s, one primes OK, the other I have to pre-fill with tank
water before re-attaching. I don't know if the second one is a lemon or if
the difference in the tubing angles makes a difference. I know they will
BOTH leak if I'm not real careful putting on the tops.