Not So Flammable Liquid Wanted

List:

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?

Thanks
Lou

Lou,

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

Jochen

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

1 Like

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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…

Frank
sxj6 85 Sov Europe (UK/NZ)
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If you havent checked for fluid inside the booster chamber, do that - pull the vacuum supply line out of the booster, get a piece of light insulated wire (or item of similar lack of rigidity), dangle it down the hole to the bottom, check for fluid. Fluid that gets down there starts to restrict the movement of the push rod, usually apparent as a hard but ineffective pedal.
jp

John,

I thought that brake fluid inside the brake servo assembly and a hard pedal were signs of a bad diaphragm inside the servo assembly.

I don’t believe that draining the brake fluid will do anything to help your brake problem. I could be wrong but it sounds like to me that you need to remove and replace your brake servo assembly to fix your hard brake pedal.

Paul

The diaphragm separates the vacuum chamber (connected to inlet manifold) from atmospheric pressure side , which is controlled by a valve operated by pushing the pedal. The fluid can only come from the rear of the actual hydraulic cylinder when its rear seal fails, and this is on the vacuum side - where it can fill up and prevent movement of the diaphragm. I have seen cases where the chamber fills with fluid and it starts to get sucked into the inlet and produce clouds of white smoke. If the diaphragm fails the booster will be ineffective and will produce a big air leak in the inlet system. This will also produce a hard pedal, and require the complete replacement of the booster, but a leaking cylinder only requires replacement of that part, much simpler. Hence the test for fluid in the chamber.
jp