This thread is a year old, but in case anybody finds this searching for a solution:

I had the same problem. Single digit temps, out on the lake, tried to use the Mr Heater for the first time in several years, and I could hear gas hissing out the moment I installed a bottle. The pilot flame would light, but not enough gas was reaching the main burner area to properly light. Plus it was wasting propane and gassing up the shanty, so I gave up and removed the bottle.

I found the cracks in the rubber washer, dug through my cabinets for a replacement and found no perfect match. I tried shopping online for one, but if you can find a replacement with outside diameter around 23 MM, inside around 8 mm, and thickness around 2 mm, congratulations, you win. I couldn’t, so tried a hose washer, and it just might work with tweaking. With a hose washer the brass fitting screws in too far (because the inside diameter is wider than the brass fitting) so the other end doesn’t reach far enough into the bottle for gas to flow at all. A spacer in the middle of the hose washer could solve that, but I found a better solution:

Remove the brass fitting from the heater, and then remove the leaking rubber washer/gasket and throw it away. Get a rubber O ring, size 17 MM OD, 2.5 MM thickness. I had them on hand from an o ring assortment. Don’t install it on the heater, but pop it in the mouth of the propane bottle. It will fit snuggly inside, and when you install the bottle in the heater you’ll get a good seal. Just remember to remove the o ring from the bottle when it’s empty and transfer it to a new bottle. I used a twist tie to attach a spare o ring to the heater handle to make sure I have one handy.

So there’s a solution that works, and o rings are readily available at any hardware store, and they’re cheap.

Hope that helps.

Paul