68' 5000 overheat ?


Wayne NJ
Have a new (for me) 68' 5000 gasser that I fixed up last year. Ran it a little pulling a disc last year and all seemed OK. I just ran it over the weekend pulling a Perfecta which is a combination spring tooth, rolling basket and leveler. I actually pulled that with my 861 diesel in the past with no problems. It was about 75 degrees and I made about 10 passes when I noticed a whiff of coolant. Then steam and whistling. I wasn't sure what it was so I just backed off and in about a minute the whistling and steam subsided. I waited a while and let her cool off put about a quart of water in and drove her to the barn. Thought it was the radiator so I pulled the nose off looking for a massive leak only to find nothing. I ran at idle for a while without the cap on and the radiator got warm and the gauge never got close to red, and saw no leaks. Shut her off and buttoned everything back up.
I have had this tractor gone over pretty good, from compression check, complete tune up, new thermostat, new temp sender, cleaned radiator, new carb etc and it does run like a top. Before running her I double checked the coolant level and it was full just over the top flat piece, so there was plenty of room for expansion. So I thought that the coolant release was from the cap letting loose, which I have never experienced before. I remember looking at the gauge and I know it wasn't in the red but in the middle. So I really don't think it was hot, but could be wrong.
So I thought I would get a new cap and checked the manual and what was on the tractor and they are both 7 psi. The current cap is also made in England so it appears it is original. I looked for a new cap and while I can find 7psi caps it seems that this site and others are selling 13 psi units. So I think I should go with the 7 but am curious as to why there seems to be a push for the 13. So need opinions as to why 13 seems to be the norm and 7 are somewhat hard to come by, and would it be wise to use a 13 ?
I will be draining and refilling with coolant and getting a IR temp sensor and checking the temp right at the temp sending unit to see what it reads when running but was wondering what else I can check trying to figure out if something is wrong or if she just had a bad day ?
Thanks, any opinions welcome

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