My 140 radiator has always leaked from the cap. I even installed a new cap and its still leaking only when I stop the car. I guess pressure builds up. Does anyone know how to fix this problem?

In my 140's case, it turned out to be the lip of the radiator. I had the o.e. radiator rodded and re-cored during the resto last year. At some point the neck became less than flat. I carefully tweaked it with pliers and a small body hammer...no more leak.
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That's not a lot of info to advise on but if you've replaced the cap with no change in results, check the sealing surface for a scratch or other imperfection which can allow a bypass.

For a 140, I presume it is a sealed system with overflow bottle...so pressure cap is located on bottle, and radcap is strictly a sealing cap...also, check pressure rating to assure it is not allowing too much pressure to build.

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