Not So Flammable Liquid Wanted


As reported earlier, I believe the lack of braking power on my 73 XJ6 at low speed may be due ,at least in part, to a vacuum leak. I would like to use a mildly flammable spray liquid to test the hose connections without making an ash of myself.

Starter fluid is out due to its extreme flammability. Is there something I could use that would increase RPMs when sucked into a leaking hose connection but is less dangerous?



the intake side is not as critical as the exhaust side, anyway you’re right to be cautious. Brake cleaner is the indicator of choice. I’ve heard even water mist sprayed around the intake will change the way the engine idles if there’s a leak.

Good luck


75 XJ6L 4.2 auto (UK spec)

An unlit propane torch, is the safest.

I have a propane “pencil” torch, which is great for heating small fasteners,
last about 3min on a gas fill, hadnt thought of using it for testing individual vacuum, but it would work well, as the gas output is very focused, but small volume

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So be it, Tony…:slight_smile:

However; the engine uses a prodigious amount of air even in idle - far beyond a leak loss. So braking effect should not vary with engine rpms as a leak indicator - manifold vacuum is a better indicator of expected braking effect.

At speed, when you lift foot to apply brakes; manifold vacuum increases to some 23’ Hg as throttle closes - increases braking effect. Idling in gear tends to lower vacuum - but coasting at low speed sort of keep vacuum at idle level.

A second consideration; the one-way valve is there to keep the highest vacuum reached inside the booster - enhancing braking at all times. With a failed one-way valve; braking effect varies directly with manifold vacuum…

However, checking for leaks is certainly pertinent - whether or not this influences braking. However; if the leak is inside the booster, it will not show up with an external leak test. A leaking booster will influence braking effects, though not necessarily rpms dependent.

A leaking booster is detectable by a constant hissing at the brake pedal rod when the pedal is held down…

sxj6 85 Sov Europe (UK/NZ)