Shrew, the Groco Model K is a manual toilet--the last of the solid
bronze "thrones" still in production in the US. Last
price I saw was about $1700...which may help to explain the $100-$300
price of
rebuild kits.
Groco Model K Groco Model K instructions
WaldPinkler...CLR
might de-scale the bowl, but not the pump or the hoses. It's
cheap enough to be worth trying. The hoses can be cleared with a 12-15% dilution of muriatic acid (available from any
hardware store and FOLLOW ALL SAFE HANDLING INSTRUCTIONS COMPLETELY!!!), but will prob'ly require more than one application. So based on your description of the condition of the system when you bought the
boat that it's only gotten worse since you've owned it, the smartest thing you can do is replace ALL the hoses and put a
rebuild kit in the toilet.
The backflow only when on holding tank is most likely due to a blocked tank vent.
TANK VENT 101
All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from
flushing.
Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from
flushing.
Do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by
cleaning out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the
deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...reconnect the vent line.
You're welcome to give me a shout via PM or
email if you need to discuss in more detail than is practical in a discussion forum.
--Peggie