Sourcing the vacuum leak

Hi guys, I bought a vacuum gauge recently to start to test the vacuum in the brakes on Lorry. SO I started at the back of the vacuum pump at the alternator. Connected the gauge and almost instantly got the maximum reading, I think on the gauge of 35 In Hg at idle. All the following numbers are also measured at idle unless specified.

Next I went to the next thing inline heading towards the tanks and it is this valve and I do not know what it is called so any clues would be appreciated. At the top of this valve numbered #1 it reads the same of 35In Hg.

Next to be checked were positions #2 & #3 which showed no vacuum at idle with or with out the brake applied.

After this valve I went to the front reservoir tank with #6 on it and this is where there is a pressure switch mounted on the way out heading to the brake booster where the slave cylinder is. This measured 25 In Hg at idle and 28 In Hg at 1000 rpm.

Finally the back reservoir measured at the same place where the pressure switch is read 24 In Hg at idle and 26 In Hg at 1000 rpm.

At positions #6 & #7 when I shut down the engine the vacuum slowly recedes so there is a leak but where and how to find it. I cannot hear any leak anywhere but my hearing is not so good after being married for so long, marital deafness has set in. I cannot use soapy water to look for bubbles as it is a vacuum not pressure. I dont know If I should just start replacing valves and pressure switches but this goes against my grain just guessing where the problem is.

With that first valve with numbers #1,2 &3 on it I do not know what #2 & #3 are for if they do not have vacuum to them. Are they just a breather for the diaphrams in the brake boosters?

Image #1 is the first valve in the line. #2 is the front tank and #3 is the rear tank.

Cobba

You have to remember when tracing leaks that vacuum is only about minus 17psi at very best and involves very little volume of air "in reserve" unlike air at 100psi. A tiny leak that will destroy your vacuum in minutes will last a long time with air.

I once saw a bloke pressurise the lines - 20 psi is enough - and do the soapy water trick on every possible joint and line.

It might be worth looking at smokers they use to trace vacuum leaks on modern car engines. These do the pressure thing but you look for smoke coming out of cracks. You see them on Gumtree for $50 or so. You can rent it out to your mates to track leaks on their BMW's.

Lang

Power booster diaphragm that has a small crack would be fairly likely to be a cause

Test under slight pressure as suggested

Vacuum isnt reliable for reasons Lang has given hence they dont use it on trains for breaking anymore

Paul

Thank you for the heads up on using low pressure. This is something already on hand in the shed and a long hose to Lorry. So long as it is under 20psi all should be good and then out with the bubbles.
As for mayes and BMW's none of my mates would have one so I would be stuck with the smoker, plus I gave up smoking about 8 years back now. So after the lawns tomorrow I will have another go with air pressure and soapy water.
Thanks again.

Ok, after running the "goat" over the lawns and had a cupps it was time to 'blow up' the brakes on Lorry. got everything ready, compressor set to 15 psi, air blower rigged to hold the trigger on and some extra clamps and pieces of hose to connect the air with. With a tin of very soapy water and a brush I started by pressurizing the first valve back to the pump with no bubbles showing. Then changed direction of pressure to filling the reast of the system, both reseviours and boosters again with no bubbles showing up anywhere. I can't remeber ever spending so much time under Lorry banging my haed against the tray and other bits and pieces. End result I have a very clean brake pipes and tanks that show no sign of blowing bubbles anywhere.
So this got packed up and I dragged out the grease gun and went front to back and lubed him up ready for the next trip out. Then finished off the new radio inside and then made a USB stick full of music to throw in when the radio stations are not worth listening to. So it was not a waste of time, a little disappointing I haven't found the leak yet but got other things done and because of that I feel it was not a waste of time.
If only there was a rally on somehwere I would have the cleanest brake system in show.

Pressurising the system may have had the effect of holding the leaking culprits closed. Using resources already available in the kitchen get the gas lighter with the little flame and hold the flame up to suspect areas. Where the vacuum is leaking the flame will be sucked in.

Since using the soapy water along the pipes and hoses and all the valves top to bottom and the pair of pressure switches and it showed no bubbles I am guessing as suggested earlier that maybe a diaphram has failed in one of the brake boosters and this is where I can see it getting tricky.
As I am not sure and only guessing that for a diaphram to be moved via avacuum in one direction the opposite side of the diaphram must push air somewhere and I think maybe that 'somewhere' is to the valve with #1,2 &3 on it where the vacuum enters at the top and goes straight ahead towards the tanks but the other 2 pipes do not have vacuum in them with or without the brakes on or off. There is a couple of big bore rubber hoses connecting one booster to the next then small pipes go to this valve then I never follwed it once it went through the chassis.
I have no chance of hearing a leak with the engine running and it doesn't hold vacuum for very long, maybe a minute and a half so I would need to be quick to get back to the boosters and jamb my head between tray, chassis and tanks.

Why not run a long hose (a garden hose may do) from the vacuum pipe to a vehicle parked 10metres or so away and supply the vacuum from there.

If it's a lateísh model car it will be quiet enough for you to listen for the leak.

Did I jinx you the other week when I asked you about your vacuum in lorry cobba?
I’ve got a new alternator with a pump on the back....

No mate, not at all. The problem was still there but I was playing with the other toy lately and now while waiting for parts had a look at Lorry and his leak.
On the bright side of things at least this is on one leak you can't see or leaves a mark on the ground.
Zuffen, even if the source of the vacuum came from a mile away my slight case of industrial deafness doen't allow me to hear certain things, Dee going crook at me for something is the most common problem for me and dare I say a slow vacuum leak would be fairly difficult. I will keep looking around as time allows as I have a challenge for myself in the shed thaty I want to sort out and now everything has arrived to do this I am playing on the lathe.
I found that it has a slight 'run out' when machining along a shaft. This as I have found is the tailstock out of adjustment and with a lump of steel, some time and a brand new dial indicator I can adjust the 0.085mm run out.