I have an oil burner that I keep having to bleed every time to get it to run. The nozzle is new and so is the filter, before I go and replace the pump is there something else I could do? Do the pumps have an internal check valve in them that could be gummed up?
There can be a hole if the lines run underground from the tank. This has happen on more than one occasion and the customer said he had checked them no leaks. Guess what they were leaking and he had oil leaking out underground. He changed every part even a new oil burner assembly before he did a good thorough check of his lines. When he found the leak and repaired it no more problems.
There is a screen in the oil pump that can get clogged. I once found part of the oil filter in the oil pump.
Check your pressures at the pump about 100PSIG
Hope this can lead you in the right direction.
* Of course you may need a HVC tech to do these checks for safety concerns.