Hello, you have my sympathy ...although I do not always agree with other experts in this case I thing Gaztech was on the right track his advice is worth review.
As you may or may not be aware, Munchkin had a relatively short run and are now out of business, with only scant parts support if you are lucky...
They had less than 0.01% of the market before going out of business, very few boiler men as a percentage have ever worked on these, then given the many failure modes, even if they had worked on one or two... the particular problem is unlikely to have been the same as yours.
The average life span of boilers in that class runs roughly 10 years, give or take, beyond that point any time or money spent in repairs is likely be met with follow on problems. If this boiler is over 5 years old, and given Munchkins history and parts problems (Munchkin was innovative, very few of their parts were off the shelf so that generic parts seen on many other boilers and commonly available are not available for Munchkin.... you might be well advised to replace it most especially if it is over 10 years old or had been used to heat domestic hot water as Gaztech mentioned.
Let me know if you would like to discuss this broader situation with me or Gaztech further... meantime I will opt out so others here can see your questions.