The liquid serves two purposes:
I believe that the composition of the liquid is mostly water + glycol, quite similar to the anti-freeze liquid in your car's radiator, but without the colorants added that should warn you against drinking it.
A good replacement kit contains not only a plastic bottle of liquid but also new rubber sealing rings to keep it in, where it belongs. It's always recommended to buy the original stuff, if you can find it.
(From: firstname.lastname@example.org). I just wanted to throw my $.02 in since I was one of the original members of the design team for the first Magnavox and Sylvania PTVs.
I don't recall the models, but essentially, there are several generations of liquid cooled/coupled PTV designs. One type consists of a set of CRTs with a liquid cell as part of that component. The other type consists of a liquid cell that directly couples the CRT faceplate to the lens.
The liquid is a solution of DI (deionized) water and propylene glycol, with a small amount of surfactant to eliminate bubbles sticking to the glass and plastic surface. Distilled water can be used. The propylene glycol is USP grade, not commercial grade for clarity reasons. You need to order this through a lab supply company and specify USP grade. If you use a cheaper grade, the solution may become cloudy. The mixture of your solution should be somewhere between 50/50 to 80/20 water/glycol. The are many kinds of surfactant which can cause cloudiness or foaming. You should probably leave this step out if you are just adding rather than replacing liquid. If you want to experiment, you might try some fluid from a photographer darkroom supply store that is used for eliminating water marks during the film drying process. Just add a few drops to 1/2 liter. Then heat it up to check for cloudiness. If it works, then you are in business. Again, if you are careful with filling the cell so that you don't mix in air, you probably don't need any surfactant.
I probably can't help you too much on the seals or gaskets. It is very dependent on the specific model. Both types of liquid cells loose the liquid over time due to vapor traveling through the silicone seals. The CRTs with the cooling cell used a special RTV for a seal. The CRT/Lens cooling cell used silicone gaskets. There is a tradeoff on how tightly the gaskets can be tightened down due to CRT specifications. Some manufacturers were also working on a expandable chamber to reduce buildup of pressure when the liquid expanded due to heat. The higher the pressure, the faster the liquid would evaporate through the gaskets.