I have a 1965 V8 rad in my car with a 195° thermostat. I run 50/50 coolant mix with a Stant 10229 rad cap (13 psi). I use an old beer can as my overflow tank. The rad was last recored in 1994 and heater core in 1986. I've done a couple of citric acid rad flushes in the past few years and my car keeps a steady temperature all year and never runs hot.

This summer, I've had to repair my rad 3 times. The first 2 times were solder-related leaks while the last repair was due to a crack (subsequently found to be 3 cracks in the same area) in the upper tank. The rad shop mechanic told me that 13 psi was excessive and was the reason that my rad had cracks in it. He recommended that I use a lower pressure cap (8 psi and not more than 10 psi).

SSDan has an excellent write-up about rad caps in Allpar: Cooling systems: the surprisingly wide range of radiator caps.

Another relevant topic here is Radiator Cap Problems

Since I have a clean cooling system and plenty of rad capacity, the only time the coolant gets really hot is shortly after the engine is shut down and I heard the coolant boiling in the engine after summer drive a couple of weeks ago. The 50/50 coolant and 13 psi cap means that the coolant has boiling point of around 261°F. A 10 psi cap would have boiling point of 255°F. (See Cooling System). I think the boiling was localized in the head and I have my doubts that the bulk coolant temperature got anywhere close to 255°.

Having had a closer look at my Stant 10229 rad cap, it looks like it is a full pressure cap (spring -loaded vent valve) with a coolant-recovery seal. I checked other rad cap manufacturer web sites and it looks like all aftermarket caps are full pressure rather than partial pressure (free-hanging vent valve) but it's hard to tell from the product images.

Does anyone still make a a 13 psi partial pressure rad cap?

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1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech

What does a Partial Pressure cap look like? (I am quite sure I have never heard of such a thing, and am interested to learn more.. headed to read SSDan's article)

Read the last paragraph on this page, or take a look at a Chrysler № 52079880AA to see if they're still a partial-pressure cap. These are 16 pounds instead of 13 ¯\_(ツ)_/¯
(read the coolants chapter of that article series—waterless coolant is multiple different kinds of awesome, one of which is directly related to your cap quandary)