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Pressure Testing Non-Pressurized Cooling System


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My '38 Chrysler Royal has a non-pressurized cooling system. There is small leak somewhere. I'd like to find it an address it. It is hard to tell where it is leaking. Could be up under the bottom side of the water pump.


I was thinking, could a person remove the thermostat, re-install water neck and put a few pounds of pressure on the system? When the system is cold and sitting. Could I maybe get an indication where the small leak is? 


I don't really want to buy a full rad pressure testing tool to use this one time. I've not had another need for it in my life so far. If I found a good deal on a used tool maybe. What I am wondering is, does anyone have a hack to build a rubber bung somehow to plug the rad fill port? Then I could put a low PSI gauge on it. Add 4 psi or so and see what develops. 

Does this seem reasonable? Looking for ideas on a bung seal it possible.


Any other tips are appreciated.





38 Radiator.jpeg

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one can safely pressure the system with the use of a bicycle inner tube....you slip this over the inlet and outlet to the radiator and cork the overflow as best possible and air up the tube once all is filled with water including the tube itself.  This can be done at the block also with the inlet and outlet hose connections at the pump and t/stat cover.....I will have to say I have not tested an early one with the top hat relief and have none to look at here at the house to advise further......plugging this could take a bit more...I test my low pressure 7 lb system in said manner.....

Edited by Plymouthy Adams
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Hey Keith, I had a slightly leaking radiator about 2 years ago. ´39 Plymouth truck, non-pressurized, also.

My "testing tool" was a hair-dryer.

I removed the radiator, drained it. I saw some green spots at the honeycomb structure. Verdigris.

Applied warm air flow to the green areas, which of course dried it, but at the same time soaked moisture through the leaking areas to the honey-comb surface. Capillary effect.

Means: dry - moist - dry - moist ... uncountable.

The 2 or 3 obvious leaks were in fact 10 or maybe even 15.

I blow-dried the radiator until the leakages stopped to get moist.

Then thoroughly cleaned them with solvent.

Applied epoxy resin to the concerned areas and blow-warmed them also -> Effect: epoxy gets a little thinner, soaks into the leaking micro gaps and hardens quicker.

Works w/o any further leakage since then.


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I ground down a tire valve stem and clamped it in to the heater hose. I had to test a heater core. Other end i clamped a 5/8” bolt in the hose. Worked slick. 
my vacuum pump also pressurizes. Its handy for small volume tests. Air pressure regulator set at 4-6 lbs would work too. 

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Thanks for the tips folks. My rad seems good. It was re-cored when the car was restored. I don't see any leaks there. Tonight I spotted a little green AF on the top of the water pump at the by-pass hose area. Tonight I pulled the head inlet thermostat neck, and the by-pass mount housing on the top of the water pump. I cleaned up the area, new gaskets and a little skim of sealant. The areas were a little pock-marked up from earlier corrosion. I may have it sealed up now.  This job was the first time I think that I was able to do a repair and not need to order any parts. I dug through my spare parts stash and found both gaskets, brand new!  Gotta love that.


I drained the block and rad. Tomorrow I'll run a cooling system flush thru it for 20-30 mins then flush everything out. I see some scale build up in the rad. Doesn't look too bad.  Hopefully the chemical flush I sourced will clean things up. The good news is the block drain pet-cock was not plugged up past it's level with rust and scale!  Not too bad in there. Should clean up nicely.


Edited by keithb7
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5 hours ago, keithb7 said:

Tonight I spotted a little green AF on the top of the water pump at the by-pass hose area.

I replaced my water pump last fall and ran the car very little after that. This spring went to look at the car I had puddle of AF under the front of the engine traced it back up and also have a leak some where in the by-pass hose, top radiator hose, heater hose elbow, and back of the water pump area.


I talked with the guys at the automotive store and they suggested using some Permatex Aviation Form-A-Gasket Sealant on the hose snouts and inside of the hoses before putting hoses on, and also not to use worm-drive hose clamps cause they don't have even clamping pressure. Another thing on the list for this spring to redo! 

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I find worm clamps to be reliable but it is necessary to retighten them a couple of times after installing new hoses. The rubber will extrude through the clamp slots until everything has settled. There are aviation worm clamps that don't have slots but they are more expensive and harder to find.

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