Lets slow down and take it step by step.

(1)First and formost never rebuild or repair a Gas Valve.
(2) Go to Sears or Home Depot :
Buy a cheap electrical multi meter that has a 24 volt AC setting( be sure the gas valve is 24 volt AC ( should be stampted on the valve.
(3)How are you overriding the thermostat?
(4)Re connect the t-stat and set it for a call for heat.
check the voltage at the gas valve to see if you are reading 24 volts ...if not the problem is elsewhere.
(5)is the bolier a atmospheric boiler or does it have a forced air combustion blower. Let me know
hbrownjr@hotmail.com

Thank you for your advice, but the problem seems to have resolved it self.I over rode the T-stat simply by increasing the call for heat above the ambient room temp.I was prepared to replace the gas valve,however before preceding I retraced all my steps.This time when I primed the gas valve with the selector on run there was a click and the familiar swoosh signaling the main burner had ignited.The only discernible difference was the pilot had been heating the thermo-couple for the past 24 hours.I tried several permutations to see if the system would operate normally.First turning the t-stat down,main burner shut down,increase call for heat ,main burner re-ignited.Used main boiler switch on and off,same result.The pilot has a nice blue flame and the thermo-couple seems properly placed.My boiler heats water which goes to base board radiator via copper pipe.

If you have a mercury bulb thermostat, you might want to check the heat anticipator setting.

The heat anticipator wouldn't keep the gas valve from energizing on a call for heat. Its in the circuit when the low voltage connection is made not before.

I finally called my plumber,after I explained everything I had done.He diagnosed and fix the problem in less than 20 seconds.He explained the initial failure may have been the thermal couple but it seems I also had a faulty damper which also has a negative feedback loop.He manually placed the damper in the open position and the system worked without a hitch.Now I replaced the damper the system is 100% functional again.Thank you all and have a wonderful holiday

Is it a fairly new boiler with an Effikal damper?

There was a recall

Here is a link that might be useful: Effikal recall

Now see you never mentioned that the system had a vent damper. Thats why no one can expect miracles in this forum by just giving the most basic of symptoms when you have a problem and this forum should NEVER be used to replace a service call to a reputable HVAC company. By trying to save a buck you can end up joining your ancestors prematurely. Having been involved in this forum on and off for several years now I often wonder if it has ever been responsible for the death of some poor sap who thought any and all repairs to their heating system can be done with the help of some dumbass who actually knows less then they do!

It`s quite correct ,I did not mentioned that this system has a vent damper nor did I mention the system had two taco circulators.My expectation certainly did not include diagnostic miracles or saving a buck.By sharing experiences we should use these forums as a learning tools.People who are more knowledgeable about their home systems can make informed decision about when it is warranted to consult with HVAC professionals and better able to understand repair recommendations.Case in point,both the thermo-couple and the auto damper function as safety switches.It is the function of the thermo-couple to disable the gas valve when the pilot is extinguished.Similarly the damper in the closed position will not allow the gas valve to open.In our system we added an eight inch fresh air vent to our utility room and hard wired carbon monoxide,smoke and fire detectors with battery back up.It is this redundancy coupled with routine maintenance that gives us a sense we will not be joining our ancestors prematurely.Last but not least.It turned out there was nothing wrong with the damper.The problem was in the wire harness at the connection to the autostat from the damper.Simply shorting the wires and re-taping it solved the problem.

I have news for you, the thermo couple is not a safety device in a sense. It keeps the pilot lit by keeping the solenoid open that it is connected to it by way of a pick coil. The gas for the main burner must pass through it to flow to the burner. This is known as a redundant gas valve. Your vent damper is also not a safety device. It is an energy saving device. The end switch which is a part of the whole assembly is a safety device. Perhaps you should take a second look and what zl wrote about vent dampers. I found 2 already that were stuck closed and the end switches failed.
BTW howtan, the point I was trying to make is that without all the information and a thorough check out of the system it is almost impossible to properly and "safely" diagnose every gas heating system problem. I being an experienced tech understand that, you being someone who thinks they could learn what we know by asking total strangers questions in a forum do not.