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Dual Circuit Brakes. Please Explain Something.


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#1 nicklouse

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Posted 10 August 2018 - 10:39 PM

after reading the other topic similarly headed it got me looking and then thinking Eh?

 

now this may just be down to some words used incorrectly.

lets take the GMC227

http://www.minispare...e cylinder.aspx

it says

 

One of the notable features is the failure switch on the reservoir cap with a yellow identification tab. 

but what does it mean. is it a failure switch or just a fluid level warning?

 

now the lovely PWDA valve

http://www.minispare...~Cylinders.aspx

 

This twin line brake valve is fitted on the front bulkhead of the engine bay from 1978 on as opposed to the single line valve which was fitted on the rear subframe

now reading this i come to the conclusion (most likely incorrectly) that if one circuit fails it is the MC that controls that and the PWDA just restricts the pressure to the rear?

 

so could i fit this MC to a single line set up with an adjustable rear pressure valve and have a dual line setup? or is the PWDA the bit that shuts off the failed end rather that the MC.

 

why you may ask? well there is a possibility i will need to change to a dual brake system as i do not have a hand brake. PS yes i know all about the KAD OMP Sparco pedal box set ups ......



#2 Bat

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Posted 10 August 2018 - 11:11 PM

Hi,

To be honest I don't think pdwa is even the correct term for the mini valve as there's no warning switch fitted to it.

I guess you could class the fluid level switch as a warning as the level will drop as the vital fluid dribbles on to the floor!

Cheers  :proud:



#3 Moke Spider

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Posted 11 August 2018 - 06:05 AM

The Yellow Tag Cap has a Float Level Switch built in to it. It's not a true 'Failure' switch, the theory of these is that if you have a leak, the level will drop and the lamp should come on. On the GMC167's they have a true 'Failure' Switch, which I promptly plug up and fit the caps from the Yellow Tag. The Failure Switch won't give a warning until you attempt to make pressure in the system (ie, go to use the Brakes) where as the Float Level will warn sooner (ie, when there is a fluid loss).

 

The FAM7821 isn't a PWDA Valve. It's a Pressure Shut Off Valve, similar in operation to the type that was fitted to the Front Rail of the Rear Subframe on the Single Circuit Cars, the difference with this Valve is that it done's just look at the Pressure in one Circuit, but both and then will shut off the feed to the Rear Brakes.

 

You can fit up the Yellow Tag with an Adjustable Valve in the Rear Circuit only, or any manner of Rear Brake Control Valves out there and it will work fine. The two Pistons of the two Circuits in the Master Cylinder are not mechanically coupled, but Hydraulically coupled. So, in Operation, the Pedal pushed the lower Piston first and the upper Piston stays put, one there is pressure built up in the system from the lower Piston, then the upper Piston will start to move.

 

So, if plumbed the way recommended for a Yellow Tag (lower Port to the Rears, and upper port to the Fronts), the Rear Brakes will come on first, once they have pressure in the System, then it will bring on the Fronts, once the pressure in the System increases to the point where the Rear Brake Control Valve (of what ever type you choose) reaches it's shut off Pressure, all Pressure in the System will then go in to the Front Brakes.



#4 unburntfuelinthemorning

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Posted 11 August 2018 - 09:25 AM

Why the hate for FAM7821?  Is it that they fail often?

 

 

In the '90s I had a problem with my rear brakes locking so replaced this part with a Rover/Unipart one and the brakes were perfect afterwards with the fronts locking before the rears.



#5 Moke Spider

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Posted 11 August 2018 - 10:04 AM

Why the hate for FAM7821?  Is it that they fail often?

.

 

With all the parts and rubber in them, there is more 'slack' in the system to take up and they make the brakes feel more spongy than they could be. The Valve is overly complex for the job that it does, it's big, yet the pipework isn't easy to get at. They are also non-serviceable, though if you really want to, they can be serviced to a degree. When they have issues, they can also be hard to diagnose.

 

There's simpler an better ways of getting the job done than to use one of these monstrosities. The other Valve that was used, though it would seem not in the UK, was the Inertia Valve, 13H7757 with the Tandem Brakes, and from a Hydraulic perspective, these are 'seen' in the system they same way a Pressure Shut Off Valve (like on the early cars) would be.

 

Like Retroman said in another thread, I can't be rid of them fast enough. Complete junk in my books.



#6 nicklouse

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Posted 11 August 2018 - 10:42 AM

The Yellow Tag Cap has a Float Level Switch built in to it. It's not a true 'Failure' switch, the theory of these is that if you have a leak, the level will drop and the lamp should come on. On the GMC167's they have a true 'Failure' Switch, which I promptly plug up and fit the caps from the Yellow Tag. The Failure Switch won't give a warning until you attempt to make pressure in the system (ie, go to use the Brakes) where as the Float Level will warn sooner (ie, when there is a fluid loss).

 

The FAM7821 isn't a PWDA Valve. It's a Pressure Shut Off Valve, similar in operation to the type that was fitted to the Front Rail of the Rear Subframe on the Single Circuit Cars, the difference with this Valve is that it done's just look at the Pressure in one Circuit, but both and then will shut off the feed to the Rear Brakes.

 

You can fit up the Yellow Tag with an Adjustable Valve in the Rear Circuit only, or any manner of Rear Brake Control Valves out there and it will work fine. The two Pistons of the two Circuits in the Master Cylinder are not mechanically coupled, but Hydraulically coupled. So, in Operation, the Pedal pushed the lower Piston first and the upper Piston stays put, one there is pressure built up in the system from the lower Piston, then the upper Piston will start to move.

 

So, if plumbed the way recommended for a Yellow Tag (lower Port to the Rears, and upper port to the Fronts), the Rear Brakes will come on first, once they have pressure in the System, then it will bring on the Fronts, once the pressure in the System increases to the point where the Rear Brake Control Valve (of what ever type you choose) reaches it's shut off Pressure, all Pressure in the System will then go in to the Front Brakes.

 perfect, it is as i expected the block on the bulkhead is just the safety shut off valve on both lines so if you have a leak you still have some form of breaking. 

 

So if they say i do need dual circuit brakes it is an easy fix.



#7 Moke Spider

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Posted 11 August 2018 - 10:47 AM

 

 perfect, it is as i expected the block on the bulkhead is just the safety shut off valve on both lines

 

 

Just to clarify this point, while the FAM7821 'looks' at both line pressures, it only 'blocks off' the feed to the rears when the pressure in the line exceeds X (what ever that is, there doesn't seem to be any published figures).



#8 nicklouse

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Posted 11 August 2018 - 10:55 AM

 

 

 perfect, it is as i expected the block on the bulkhead is just the safety shut off valve on both lines

 

 

Just to clarify this point, while the FAM7821 'looks' at both line pressures, it only 'blocks off' the feed to the rears when the pressure in the line exceeds X (what ever that is, there doesn't seem to be any published figures).

 

right, got you seems to be a dumbing down of the older set up. (cost saving).



#9 Pete649

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Posted 11 August 2018 - 05:12 PM

So, in Operation, the Pedal pushed the lower Piston first and the upper Piston stays put, one there is pressure built up in the system from the lower Piston, then the upper Piston will start to move

 

Just to clear up my understanding of how the GMC 227 works, what happens if there is a problem with the rear braking system so no pressure can be achieved?



#10 Moke Spider

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Posted 11 August 2018 - 06:42 PM

 

So, in Operation, the Pedal pushed the lower Piston first and the upper Piston stays put, one there is pressure built up in the system from the lower Piston, then the upper Piston will start to move

 

Just to clear up my understanding of how the GMC 227 works, what happens if there is a problem with the rear braking system so no pressure can be achieved?

 

 

Good point raised and one I didn't cover.

 

In normal operation, as described, the coupling is Hydraulic, however, in a failure mode, one of the Pistons will have an extension that will mechanically couple the two pistons roughly about 60% travel of the Lower one to the Top one. The Top Piston also has a similar extension for operation should that circuit fail.

Sorry, I don't have a cut-away diagram of the GMC227, however, this is the internal parts, you can see these piston extensions here;-

 

jQ2fKW8.jpg



#11 nicklouse

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Posted 11 August 2018 - 07:04 PM

mmmm forgot that i have put the wiper in that area

AbRpOzo.jpg

 

might have to get a scrap one and see if it fits.



#12 Moke Spider

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Posted 11 August 2018 - 07:12 PM

Nick, for your car, I'd be more inclined to go to a Balance Bar and Twin Cylinder Set up - if there's room. It will give an added adjustment to your system.



#13 nicklouse

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Posted 11 August 2018 - 07:18 PM

Nick, for your car, I'd be more inclined to go to a Balance Bar and Twin Cylinder Set up - if there's room. It will give an added adjustment to your system.

i have enough adjustment with the rear adjustable at the moment it is just if i need to have dual or two braking systems. hand brake will be hard. and to be honest the dual setups are in my exes over priced. but i know what you mean. 



#14 Mini Manannán

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Posted 12 August 2018 - 11:55 AM

Why the hate for FAM7821?  Is it that they fail often?
.

 
With all the parts and rubber in them, there is more 'slack' in the system to take up and they make the brakes feel more spongy than they could be. The Valve is overly complex for the job that it does, it's big, yet the pipework isn't easy to get at. They are also non-serviceable, though if you really want to, they can be serviced to a degree. When they have issues, they can also be hard to diagnose.
 
There's simpler an better ways of getting the job done than to use one of these monstrosities. The other Valve that was used, though it would seem not in the UK, was the Inertia Valve, 13H7757 with the Tandem Brakes, and from a Hydraulic perspective, these are 'seen' in the system they same way a Pressure Shut Off Valve (like on the early cars) would be.
 
Like Retroman said in another thread, I can't be rid of them fast enough. Complete junk in my books.
What do we put in its place Chris?

#15 nicklouse

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Posted 12 August 2018 - 12:03 PM

 

 

Why the hate for FAM7821?  Is it that they fail often?
.

 
With all the parts and rubber in them, there is more 'slack' in the system to take up and they make the brakes feel more spongy than they could be. The Valve is overly complex for the job that it does, it's big, yet the pipework isn't easy to get at. They are also non-serviceable, though if you really want to, they can be serviced to a degree. When they have issues, they can also be hard to diagnose.
 
There's simpler an better ways of getting the job done than to use one of these monstrosities. The other Valve that was used, though it would seem not in the UK, was the Inertia Valve, 13H7757 with the Tandem Brakes, and from a Hydraulic perspective, these are 'seen' in the system they same way a Pressure Shut Off Valve (like on the early cars) would be.
 
Like Retroman said in another thread, I can't be rid of them fast enough. Complete junk in my books.
What do we put in its place Chris?

 

you can just fit the limiter on the rear as per the single line cars, or an adjustable version.






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