[Uhehner]

We own a 2002 3800. I went to go do an overboard pump out on my holding tank and the handle on the seacock (Y valve) snapped off.
Can I just replace the handle and be good to go or do I need to replace the whole valve.  How would I do that if the tank is half full?  
Thanks,
Pbud

I have had multiple valve handle failures on my boat. It's an '88. In each case the steel handle on the original valve rusted and weakened. Trying to find a replacement handle that fits was impossible. I replaced the whole valve with West Marine valves with heavy stainless steel handles and the problem was solved. 
You could try opening the overboard valve using a vise grip pliers for a handle. Then flush the tank and replace the whole valve.
Keep a short piece of PVC pipe on board to give you added leverage when cycling the valves. This should be part of your yearly maintenance.
Capt. George