slant/fin liberty l-30h-p burner will start if i press relay and hold it

I would need the model number and serial number of the unit to help you on this. However, I don't know if this is an oil or gas fired unit. I supect you have a limit switch that is defective. On an oil fired unit there is a terminal strip. If you put a jumper wire from the white wire to the black wire and the unit fires then you have a open limit switch. If it fails to fiire then you have a broken wire going to one ofthe two limits or a defective safety control and (or) posibality a bad cad cell.