What’s that for?
Hand brake and Broms brake…
The brohms brake is more a safety feature when pulling away, if the pressure isnt high enough it wont allow the park brake to release, if a leak was to occur in circuit 1 or 2 (front or rear) then the park circuit is protected by a one way valve on the 4 way valve on a B10, if a brake chamber park section fails and leaks out all the air then the park will come on. Technically the Brohms wont apply the brake or have any effect on the park brake other than release it after building up the air pressure,or if the pressure is too lowdue to a leak its not the brohms brake that applies the handbrake, the pressure in the circuit wont be high enough to overcome the spring in the chamber and release the park brake. Its not a third type of brake, its basically a valve that opens when the pressure is high enough.
Really good explanation. Nothing to add, except, “thankyou”.
Pete 
Thats probably too good for me 
So if this is a protective thing that comes on and off on its own why do we have the handle at all?
I suspecy other lorries have a similar orotection feature but no additional handle?
Still confused
Very good explanation,sombody has to educate the young UN’s lol,lorries with those systems on them were easy,and enjoyable to drive,aaaah them were the days
Priest:
Thats probably too good for meSo if this is a protective thing that comes on and off on its own why do we have the handle at all?
I suspecy other lorries have a similar orotection feature but no additional handle?
Still confused
It’ll let you knock it off when the air is built up sufficiently enough,if you have a major leak it won’t let you release it,the Brohms brake will only “activate” iirc it won’t self release hence the need for the handle.
You’ll find that after swapping a trailer it may activate so always try that after checking the trailer brake because you’ll look a plum when you call out for a assumed breakdown because the wagon won’t go anywhere when you release the handbrake 
I first came across these in a Volvo coach I purchased in the 70’s when I was a coach operator. Volvo were uncommon at the time and I certainly didn’t know my way around them. Ministry man came to my garage (as was normal when a coach changed hands), pumped the air down (to ensure warning devices were working correctly) but then couldn’t get brakes to release with the air built up. Ended up with a Prohibition Notice cos neither of us knew what we were doing!
That is the limit of my education with these - but it’s been enough to keep me out of trouble ever since! Steep learning curve - and expensive.
As far as I know, only Volvo have this system. I can only assume that other manufacturers have safety devices built in but without a little knob to shove in. I’m not a mechanic so I’m going no further with this one. I did once ask my mechanic “why?” and I’d just about lost the will to live by the time he’d finished explaining!!
Pete
Peter Smythe:
I first came across these in a Volvo coach I purchased in the 70’s when I was a coach operator. Volvo were uncommon at the time and I certainly didn’t know my way around them. Ministry man came to my garage (as was normal when a coach changed hands), pumped the air down (to ensure warning devices were working correctly) but then couldn’t get brakes to release with the air built up. Ended up with a Prohibition Notice cos neither of us knew what we were doing!That is the limit of my education with these - but it’s been enough to keep me out of trouble ever since! Steep learning curve - and expensive.
As far as I know, only Volvo have this system. I can only assume that other manufacturers have safety devices built in but without a little knob to shove in. I’m not a mechanic so I’m going no further with this one. I did once ask my mechanic “why?” and I’d just about lost the will to live by the time he’d finished explaining!!
Pete
so thats what traumatised you in favor of ivecos instead of proper trucks…
be a man at the end of your life cycle eventualy and buy one proper truck to overcome your boogies and go with dignity and legacy combo…
P.S. Don’t drink and post… 
That makes sense now, great responces, thank you all
Broms, or Brims, is Swedish, Norwegian, Finnish, and maybe other Nordic for BRAKES. Simples!
This is, in fact, a very simple safety device which has only, in my experience, ever been fitted by volvo, who introduced it with the launch of the F7/10/12 in the late seventies, and should be a compulsory fitment on all trucks.
It is among other things, a blocker valve which senses low pressure in the braking system, and automatically closes, preventing the handbrake from being released until the pressure is built up, and the driver, in his cab, presses it in. (It will not stay in until sufficient pressure is available.) What is the value of this?
A number of years ago, a man I knew was killed as a result, partly of his own carelessness, (which of us has never been careless?) and certainly because the vehicle which killed him did not have this system fitted.
This is what happened.
He drove a tipper with the council roads team. He was always up and at it early, and was first to the yard from which he worked most days. One cold winter morning he got into the truck, to find the batteries were low, and it wouldn’t start.
The yard was on quite a steep slope, so he tried to bump start it down the hill, but the air pressure had dropped enough overnight that the handbrake would not release.
Not being a man to fall at the first fence, he found a truck that would start, drew it across the front of the other and used jump leads to get it going. While the first vehicle sat idling, he rolled up the leads and was busy putting the battery cover back on the donor vehicle when the other truck, having built up air, rolled down the slope and crushed him to death. You see, he had forgotten to reapply the handbrake after his failed attempt to bump start the engine.
Had it been a Volvo, fitted with a blocker valve, it could not have happened.
This valve has other very useful functions as well,but jeezuz, any body reading a post this long will be losing the will to live.
Maybe some other time.
Be careful out there y’all!!