Lorries Brake Vacuum

Today I had a little time to investigate this again. Last episode I was not sure what or if there was any vacuum so I bought a gauge and it was reading good vacuum and quickly. I now forget that number. So at start up after not running for a couple of weeks it takes around 12 - 15 seconds for the vacuum light to go out. I do have brakes.
Now on the dash board there are 3 lights for brakes, 1 is for park brake, 2 is for vacuum and the 3 is for squeeze the cheeks of your bum together and hang on as you have no brakes. With #3 if it is alight then there is the accompanying squeal from the alarm.
So my logic was that a switch maybe stuck and causing this problem. SO on the vacuum tanks (both of them) there are small switches with 26v going to them. These connections are clean and when either or both are disconnected with the engine running the brake warning light is still on and if you release the hand brake then you get squealed at.
So to the next switch which is directly from the vacuum pump on the alternator.
Line #1 is directly from the pump and is making vacuum, then continues to the tanks.
Line #2 goes to I don't know where but back through the chassis.
Line #3 comes from the booster and is possibly the back side of the diaphragm and no vacuum there at all but I have not tested this with the foot brake applied.

So this switch has been removed as it has a solenoid as the main working part I thought maybe this is stuck. Stripped it on the bench and it was clean inside the small valve that operates up and down was free and that valve has 2 small round flat rubber seals to seat against the seat to redirect vacuum to line 1 or 2. those small round rubber seats have small indentations on them but nothing excessive.
Next I wanted to test the power to the switch but with the engine off or running there appears to be no live wire here. AS I have no idea as to what exactly this switch does or when power should or should be here.
Any help will be gratefully appreciated.

“Curiouser and curiouser!” Cried Alice (she was so much surprised, that for the moment she quite forgot how to speak good English).

Does the switch get power when you put your foot on the brakes

Guessing line 2 goes to a vacuum tank as trucks are required to have reserve vacuum if the motor is not going

Paul

Will have to find a stick to test if that is the case.
Line one is from the vacuum pump and continues to both tanks via a "T" piece. Line two will have to be chased later to find out where on earth it does go.
Thanks for the replies.

PDU, Your quote from Alice in Wonderland is good but I think that Billy Bunter (The Owl of the third Remove) would have said "The curiousness is terrific."

I expect that given your history you would understand the reference but probably no one else here does?

My guess is;

The wires are part of the alarm circuit. Which requires a sequence of events (switches) to be open/closed to trigger.

The "solenoid" is a mechanical spring loaded vacuum switch which switches in emergency vacuum if the main vacuum is lost.

I couldn't stand that obnoxious fat Owl of the Remove, least of all the old black and white film portrayal of Billy Bunter, sorry Morris. :dry:



Although I have watched the odd episode or two the whole Greyfriars scene never appealed to me, in book, cartoon, or comic form - perhaps I was just a public school boy at heart? In fact I could stomach the over acted Andy Hardy easier, and he was also obnoxious too!

I jambed a stick from seat to brake pedal, confirmed brake lights were on and with the engine running. Tested the wires for power and still no electrickery at the connection. So at least that is now a known thing.
Then I was having a cuppa and thought about something.
Well kick my arse 'til my nose bleeds. The brake fluid level was near the minimum level. Guess what fixed the problem?
Sometimes it makes me wonder where my mind is at. I still want to follow the #2 vacuum line just to see where it goes but we are having thunderstorms now so will wait until next time.
Thanks you to all who have tried to help this nutter out.

So were did the brake fluid go ?


Paul

I wish I knew that answer. No oil on the floor of the cab. No oil stains on the inside of the tyres or wheels. Over the years I have had hydraulic brake lines replaced and the last time they were doing that I confirmed with them that ALL lines have been replaced due to rusty lines and they said yes.
It cant disappear by itself and your question is a good and valid one. Even Dee asked the same question as did I when I discovered it. I guess this may be an ongoing issue until I find a possible line that was not replaced, then poop will hit the fan.