[Bob Carriere]

Your HUP came with the engine that you have but you are not sure if it is original or not.......[I]You may want to do a search on the serial number stamped near the distributor........

I stand to be corrected by any one else but the picture you showed of your water pump also shows a cab 11 throttle linkage that I have never seen on a HUP........ might be clue that the engine was once changed.

On the steam line........ it has been my experience that all CMP have a steam line that runs between the rear of the engine head between 5 and 6 cyl. and connects to the elbow filler neck of the radiator...... my GM Chev book learning is the same they all have them. I understand that under severe road conditions up and down gullys ...... air can be trapped in the high spot at the back of the engine head so a simple remedy was provided. If one is not present on your truck you may need to install one if for no other reason but to get ride of the air lock you are currently experiencing.......
Having said you need one it is possibly more than a home job tobe done. You will need to have a threaded fitting soldered to the radiator tank....or if you look carefully at your filler neck of your rad right at the ebnd is where you may have had a hole. In any event any brake line that will match up to the fittings you will use will be sufficient. On the engine head where the existing temperature bulb is attached you need a brass T fitting...... one opening will be for thesteamline one opening will fit in the block of may need a reducer brass fitting and thte top holeof the Tee will take the original bulb. You may have some searching to do to find a brass fitting that will accept the original square four sided nut of the temp. bulb nut. I did not want to trust the original CMP gauge onmy truck with an expensive fresh rebuilt engine and opted for an after market mechanical temp. guage........ it registers at 165 F and sometimes at 175 when being driven hard. One note on thermostat temperature ratings........ they come from 165 to about 210 for pressurized system....... all they do is promise to open up full at the set temperature....... they will not prevent a dirty engine block or plugged radiator from having the engine run over hot and boil over.... it only shows the temperature at which it is designed to fully open. On freshly rebuilt Chev 6 cyl driven in the street of Canada I would consider a 185 or 190 thermostat to make sure the it keeps the engine hot up to the indicated heat rating due to the 6 months of cold weather.....but for my 261 truck engine I opted for a 165 thermostat as I know how hot these engines tend to run during the Summer....... I have driven my truck hard at sustained 3000 rpm aned never recorded anything over 175..... another trick is to keeping it cool is to
increase the engine oil volume...... I did away with the original CMP by pass oilfilter can and replaced it with two PH-8 screw on oil filters that gives my at least and extra liter of oil to carry the heat away. You may want to consider buying a new modern mechanical temperature guage just to run some test for a few hundred miles and go back tothe original once you are satisfied it does not run hot and reinstal the original dash temp. guage of the CMP

Sorry break time...... wife did her famous Linguini Carbonara Roma for supper..... just out of this world......

Just out of curiosity is your dash the civilian rectangular guage set or thelater military round gauges??? If you have the original rectangualr civilian dash it may indicate an ealry production HUP with whatever isntrument cluster was regualrly available....... not a guarantee has I have seen Autralian 1944 C60 built with civilain guages.....

One civilian truck 6 cyl engine such as the 261..... made in the late 50s GM drilled small holes in the heads between the 2nd and 3rd and 4th and 5th cylinders and called them stream holes.......

Now this one is long enough......look forpart two.