Brake problem...

[Pinger]

Got a brake problem with my 1999 Suburban - 5.7l with HydroBoost - that knowing a couple of things might help me get to grips with.
Firstly, the more forward, smaller, part of the reservoir is for the rear brake circuit - yes?
What causes the light on the dash to illuminate and, how does the system detect low fluid level?

A quick outline of what happened. Set off on Tuesday and the pedal was softer than usual. Had to stop for fuel 10 miles from home (but with reduced braking power) and checked the fluid level which was fine. When I set off again I noticed the light on the dash illuminated. 60 miles later and the fluid level had dropped - the forward, smaller part - requiring a top up. A little later I noticed it leaving drip marks on the ground and some time later still, a small pool of fluid underneath just forward of where the fuel filter is located. Throughout all of this I had brakes but not the full complement - nowhere close!

New pipes throughout were fitted last year and I had some difficulty getting good sealing on the rearward pipe not least as despite Amazon's claim of it being cunifer, it is actually copper coated steel so my initial assumption was that a union was leaking. But when had a chance to look today I can't locate specifically a leak point. The unions are bone dry and although there is what I think is brake fluid on the electrical cabling that runs rearwards from the engine bay on the driver side (it's hard to tell it from the general salty winter moisture) I can't provoke it by pumping on the pedal.
I'm starting to think the pipe itself has split however unlikely that is with it less than a year old. Another oddity is that this points to failure of the rear circuit, but it did not feel like what braking I had was from the front. There's no sign of any fluid escaping the drums and the wheel cylinders are only a few years old.
Before (when the weather is conducive) I start ripping out the pipe to inspect for damage, is there anything anyone can think of that I've missed? I'm perplexed as to why the dash warning light is on - what is it reacting to?
TIA.

 

[HotWheelsBurban]

Sounds like maybe a master cylinder or hydro boost unit issue....
Check around the seam between the master cylinder and the front of the hydro boost unit. They are prone to leak there.
I know on the vacuum boost units, if the MC goes bad, it can leak inside the booster, filling it with fluid, but making it not apparent that it's leaking. I presume the same thing can happen on a HB system but not entirely sure.
Are there any leaks from the calipers or wheel cylinders?

 

[Pinger]

Sounds like maybe a master cylinder or hydro boost unit issue....
Check around the seam between the master cylinder and the front of the hydro boost unit. They are prone to leak there.
I know on the vacuum boost units, if the MC goes bad, it can leak inside the booster, filling it with fluid, but making it not apparent that it's leaking. I presume the same thing can happen on a HB system but not entirely sure.
Are there any leaks from the calipers or wheel cylinders?

New MC a few years back and all the pipe work in the engine bay is bone dry - ditto the HB unit. Calipers, flexibles and all pipes to them also bone dry. No sign of fluid around the drums either.
The fluid loss was around a pint over the day and for sure it would be obvious at the drums if leaking there. Only that front part of the chassis/frame where the cabling is is wet and that is where the pool on the ground was. I fully expected it to be the union located there to be the culprit but it isn't. Starting to look like a split pipe and I'm wondering if frame flex has damaged it.

 

[1998_K1500_Sub]

What causes the light on the dash to illuminate and, how does the system detect low fluid level?

IIRC that light is triggered by an imbalance in the pressure between the front and rear brake systems.

There’s a switch near the MC that’s connected to both systems. A pressure imbalance between them pushes the switch’s internal contacts so that contact is made and the electrical circuit closed.

I don’t know that it actually detects low fluid, it may simply detect the result of low fluid.

 

[Pinger]

IIRC that light is triggered by an imbalance in the pressure between the front and rear brake circuits.

There’s a switch near the MC that’s connected to both systems. A pressure imbalance between them pushes the switch’s internal contacts so that contact is made and the circuit closed.

I don’t know that it actually detects low fluid, it may simply be it detects the result of low fluid,

Thanks. That explains the light.
Looks like it's nothing more than a split 9 month old pipe.
Annoyingly, I only ended up with that pipe as I heeded the warnings that copper pipe wasn't up to the task. I've never had copper split - ever.
Freakishly - I was stood around outside at the weekend looking at my Burb and for no apparent reason what popped into my head was how to sabotage brakes by cutting a pipe. Not like in the movies where they cut right through - a driver would notice that before joining the road - no, it would have to survive a few miles before failing. Stupid ideas I dismissed and forgot about - then Tuesday...

 

[1998_K1500_Sub]

I've never had copper split - ever.

I thought it was unwise to use copper in an automotive environment… the copper can’t tolerate vibration, so cracks and leaks ensue. It’s definitely a no-no for use in fuel lines.

how to sabotage brakes by cutting a pipe.

Thank goodness for dual circuit brakes.

We’ve got a 1964(?) Chevy C60 single axle straight trunk and it’s got a single hydraulic braking circuit I’m amazed such a stupid practice lasted so long. Dual-circuit systems entered the mainstream in… 1968?

 

[Pinger]

I thought it was unwise to use copper in an automotive environment… the copper can’t tolerate vibration, so cracks and leaks ensue. It’s definitely a no-no for use in fuel lines.

Yet the copper pipe running the LPG (propane) from the tank to the engine bay was perfect 20 years after being fitted originally...

Thank goodness for dual circuit brakes.

You bet!

We’ve got a 1964(?) Chevy C60 single axle straight trunk and it’s got a single hydraulic braking circuit I’m amazed such a stupid practice lasted so long. Dual-circuit systems entered the mainstream in… 1968?

One of my observations from this debacle is the need to maintain brakes in tip-top condition. Dual circuit in theory gives 50% braking if/when one circuit fails. Reduce that to 25% if one of the remaining brakes isn't working as it should.

 

[someotherguy]

On these trucks there's no specific device that detects low fluid, just the symptoms that result from it. GM fullsize trucks didn't have a sensor for brake fluid level until GMT800's

Richard

 

[1998_K1500_Sub]

Yet the copper pipe running the LPG (propane) from the tank to the engine bay was perfect 20 years after being fitted originally...

Perhaps I mis-understood the issue with copper on automobiles.

The issue seems to be that gasoline can cause the copper to become brittle...

copper fuel line.. bad idea?

was considering running copper fuel line(appearance reasons), but I have heard varying opinions on this..one said that the copper would react in some...

www.jalopyjournal.com

Use copper with propane: Not a problem.

My memory (what's left of it anyway) was that copper wouldn't tolerate the vibration, and develop a leak, so don't use it for fuel. I was told this a LONG time ago... perhaps I'm not even remembering it correctly, Lord knows that happens.

So you made me check, and after a casual investigation I learned something

I still won't use copper on a vehicle tho, old habits die hard.

 

[Pinger]

A quick update seeing as I've found the source of the leak. A brand new in July last year copper coated steel pipe is porous just forward of where it is held against the frame rail by a plastic bolted on clip (forward of the fuel filter). I'll know better (maybe) when I remove it if it has corroded or what has caused its porosity but I don't think it is wear (eg, rubbing against frame).
Struggling to believe that an eight month old pipe has failed but that is what has happened.