I haven't, but, the pressure test was conducted separately and it showed a leak so unless there is a problem with the gasket on the reservoir such that the device cap ALSO failed, I'm afraid that this otherwise awesome and easy fix isn't happening in my case.

The pressure test doesn't test the cap as the testing pump goes in its place.

Agreed, however, there was a leak detected in the system despite removing the cap from the system. If the cap was the cause, the pressure tester would not have confirmed a leak unless both were faulty, and I doubt this no matter how much I prefer it.

Could be the head gasket, when mine failed there was no evidence of a problem, a little bit higher temps. But I was getting enough over pressure to blow coolant out the overflow line but never saw it until the issue got worse.

Try a piece of fuel line on the overflow and stuff the open end into a plastic bottle, if you are over pressure or the rad cap is failing and not holding pressure the coolant will end up in the bottle

Is there any way that a head gasket failure would NOT result in the presence of exhaust gas in the coolant (when using a CO2 color-catalyzing tester for 25minutes at idle), or NOT result in the presence of mayonnaise/milkshake consistency oil in the dipstick or oil cap?

I used this: Block tester BT-700 - rented from Autozone

And the oil on the dipstick is typical dark caramel/black.


Also, FWIW, there's a pressure overflow tube that runs down the length of the coolant expansion tank, dumping to the ground. It is in the back of the tank on the right side when looking from the front. There have been no UV-bright pools of coolant around that area after driving for at least 1 hour in 75 degree weather.

You reused the clamps and tubing, as does nearly everyone, and there is evidence of a prior leak on the hose. Sometimes, there is a little debris in the hose which prevents a good seal. You can put tissue paper around the connections and run the engine while parked, then check to see if there is any dampness on the tissue paper. If so, reconnect the hose after cleaning it and check again.
There aren't that many places to have a leak and no liquid hits the ground, the throttle heater plate is such a place and is easy to check.

Agreed, and I def thought I had found it when it was replaced.

Likewise, I'd have expected to see fluid and bubbling while I was staring at it while it was compressed to 20.p.s.i ...

I will do the tissue paper test and report back.

Meanwhile, where is the evidence of prior leak you see?
https://cimg4.ibsrv.net/gimg/landrov...86eecf4f13.jpg

Your diligence is good, however don't think that more than one leak could be happening at once. Not with these trucks. Typically when one part of a system is old enough to fail others are not far behind.

That was exactly my situation zero evidence, no exhaust gas , no milkshake nothing but over pressure. My Disco would idle at 194 all day, it only over pressured after a hi way drive of over 30 minutes at 40+ miles an hour. And then only when I stopped and shutdown, it drove me nuts for week. Spoke to a mechanic I trusted, described the symptoms and he said head gasket just starting to go and he was right.

That does not mean this is your issue, the lower intake manifold can too you need an inspection cam to see under there as it tends to leak at the back

Nuts.

Thanks. So you would stop, shutdown, and *then* you would be able to see coolant leaking from where exactly? I have a crappy flexicam, will test this, appreciate tips on where to stand/lay and when/how to maximize checking location placement.

Seems like pumping the pressure up to 21+ p.s.i. on a non-running engine would be the best way, unless there is learned argument from experience that this will not reveal the results if the engine is not already hot or currently running (high internal compression pressures).