Like most older coaches, at some time you are faced with what appears to be a major repair/rebuild job. A toilet that won't hold water is one of those jobs that requires a major overhaul. The following procedure outlines the task for a Thetford Aqua Magic model GH that was factory installed in my coach. You will need the following:
Here's an exploded diagram of the toilet so you can see all the major sub-assemblies.
The repair package consists of all the seals you will need to replace in the toilet. The float assembly is needed to repair the vacuum breaker (#3 in the diagram). This part is extremely important and prevents a backflow condition that can contaminate your fresh water supply. You should always check/repair this when the toilet is removed. The cost of the repair package and breaker float will run about $40.
The diagram should give you a good idea of the toilet and it's major sub-assemblies. We need to get access to the mechanism assembly (#8 in the diagram). The toilet MUST be removed from the coach in order to rebuild it. Locate the 2 hold-down bolts that hold the toilet securely to the floor and remove the nuts. One is located behind the pedals and can easily be seen. The other is located in the rear and is best "found" by feeling around with your left hand behind the toilet for the opening back there. Once the 2 nuts are removed, gently lift the toilet up and to the LEFT just enough to clear the bolts that the holding nuts were attached to. Slip a 5/8" open end wrench behind the toilet and remove the water line at the compression nut. Now you can remove the toilet to your workshop for the rebuilding process.
Turn the toilet upside down and remove the screws holding the mechanism assembly to the base of the toilet. Label and then remove hoses to the assembly. Make a drawing for reassembly beforehand of the pedals and the spring attachment, then remove them from the mechanism. There is a little clip that hold the pedal assembly to the blade. Pop this clip, pull the pin holding the pedals and the whole thing will come apart. Remove all screws to the cover of the mechanism. At this time, clean everything real good especially the blade (see the diagram provided on the mechanism repair package). Remove all the old seals and replace with the correct ones from the repair package. Use the PLUMBERS GREASE to lubricate all seals during assembly. Do not use any petroleum based lubricant on the seals, it will destroy them. Follow the directions carefully for the perimeter seal and reassemble the mechanism. DO NOT lubricate the perimeter seal. Reassemble the pedals to the mechanism replacing any worn parts. Use a new c clip to hold the pedals to the blade (local hardware store 10 cent item), replace the springs and set the assembly aside.
Now we need to replace the float assembly in the vacuum breaker (#3 in the diagram). Remove the pins holding the seat and seat cover by popping the c clips, then work one pin at a time into the center open groove to remove it (#2 in the diagram). The vacuum breaker can now be removed by squeezing the locking tabs on the assembly, pulling the assembly towards the rear of the toilet clearing the seat hinge points through the holes and lifting it out. You may find that removing the water lines makes this easier. Once the breaker assembly is out, remove all the cover screws, lift the cover off and remove the float. Replace the float with the new one and replace the cover and screws. Reinstall the breaker assembly to the toilet and reinstall the seat and seat cover, pins and c clips.
Reassemble the mechanism (#8 in the diagram) to the toilet, reattach all water lines and check that the pedals do not bind. I replaced all my internal water lines with new ones at this time. Replace the old flange seal with the new one and reinstall the toilet in the coach. Attach the water supply line first, then place the toilet on the floor bolts and attach the hold down nuts. Final check everything, then leak test. The whole procedure took me about 2 - 3 hours because I went slowly and diagramed stuff as I removed it. No leaks on final assembly and the thing held water! Thanks to Chris Bryant, the resident RV Specialist on rec.outdoors.rv-travel, for supplying the Thetford diagram and helping me to understand how to repair it.