Vacumm leak

We are away with Lorry our FC144 Hino and find that it is loosing vacumm from the brakes fairly quickly. If I shut him down for a minute then start again it takes a short while to make vacuum again so the warning light goes out.
What is the best way to find out where the leak is?
Normally does not leak vacuum this quick.

Are you sure it's vacuum and not compressed air?

If vacuum, it probably only goes to the vacuum assist on the master cylinder.

The most obvious place to look is at the vacuum pump which would most likely be attached to the alternator.

Then hoses, then the vaccuum assist on the brake master.... oh and it may have a check valve which is not sealing properly..... I'd check the check valve first.

Sometimes, you can get a replacement vacuum pump, or if not, you might be able to overhaul it; the easiest is to replace the alternator and pump in one go.

If the brake vac builds up ,the pump is OK........the leak may be a perished hose,or a check valve,or the vlaving in the booster....(one or two?)...........most systems also have a filter cannister to stop brake fluid or dirt getting into the engine.......it may also be the booster piston,if you have a vac uum suspended system.........this means the booster has vac either side of the piston/diaphragm,which is applied by allowing air into the push side,when the brake pedal is released,the valve then applies vacuum to the cylinder.........so you can see air can leak in either the valve or cylinder

an exhaust brake will also be vacuum operated,so check that system too.

Thanks for the replies. Definately vacuum system on Lorry. Vacuum does build up ok just does not hold it for very long when shut down. I found a big problem with the flexible hose from brake fluid resevouir to master cylinder. It had rubbed on the brake pedal and as I touched it it then split. So replaced it with a longer hose for ATF oil to get home on. Have fitted a new exhaust brake unit but never checked any hoses. So when we get home I have a job to do.

the booster will have an air inlet with maybe an air cleaner on it........if you can disconnect the air cleaner and get a round pipe ,stick your thumb over it with the engine running..........there should be no suction against youre thumb.........if there is suction the booster valve is leaking........the air valves often get a dirt /dust &oil buildup on them.

This is the mongrel flexible line from the brake fluid reserviour to master cylinder and can see where the pedal rubbed through. This is not the cause of my vacuum leak but it was a shock to find but glad we found it before we had an epic fail. So as soon as I can I will go searching all the flexible lines first and find these check valve and air cleaner and keep searching until sorted. Not being familiar with all this stuff, yet, is there any particular place to start looking for the check valve/one way valve on Lorry?
Thanks in advance for any extra help.

Spray some aerostart around and the engine will gain revs when it sucks it through the leak

Pretty sure that only works on petrol motors not diesel with a vacuum pump.

oliver1950 wrote: Pretty sure that only works on petrol motors not diesel with a vacuum pump.

Correct