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MG MGA - MGB Dual Circuit Master with Modified MGA Cage

Has anyone ever modified the Cage or fabricated a new one to take a late MGB or other Dual circuit brake master cylinder and separate clutch master cylinder?

The pushrods are quite long, it should be possible to use up some of that space, say perhaps 30-40mm
If a new cage was fabricated with the master cylinders further forwards then we would eliminate the need to cut a hole in the fire wall to fit dual circuit brakes.

Has anyone tried this.....

This is the traditional approach I've seen in the past.

http://www.autovintagery.com/images/av6.jpg

Cheers <MARK>
Mark Hester

Mark, I like that pic. Is it your setup? If so, will the wiper motor fit in there? How about the heater?

Fred
Fred H

Fred,
No that is my plan B.

Contact Auto Vintage if you have any questions.

I'd rather know if it was possible to build a cage to fit the MGB M/Cylinders then perhaps have a batch made. As I already have a Dual Circuit MGB Master Cylinder.... Not 100% if the bore size is correct though.

Thanks
Mark.
Mark Hester

Nice set up.

It wouldn't be too difficult to incorporate brake balance too.

Nigel Steward

I hate to be the odd one out, but I am not all that keen on the setup shown.
With failure of one circuit the pushrod would go fully forward in the master cylinder and the crossbar would be at about 45 degrees. The other pushrod would have a side load on it, not acting concentric to the master cylinder bore. In the worst case half the force would be applied to the piston and half the force would force the pushrod against the side of the master cylinder bore.
I also do not see it as good design for a front/rear bias system, for the same reason. If you screw the crossbar to one side the pushrods would be at an angle to the master cylinder bore, but they would be parallel and therefore would not change the bias, but would make both the front and rear braking forces less effective. All a bit strange!
The tandem dual master cylinder is the way to go, at all times the pushrod is concentric with the master cylinder bore.


Mick
Mick Anderson

I'm with Mick too, That may look cool, but the pitfalls are obvious.

What I really want is a short enough dual circuit master cylinder, so we don't have to cut a hole in the fire wall to make it fit.

The answer is: There just aren't any short enough.

Thus a modified cage is needed.

Lets face it, the cage is the cheep bit.... Right !!!

Anyone got an MGB Dual circuit master or Other and the pedal box out ?

Mark.

Mark Hester

Just put lateral thinking cap on. Could the existing MGA MC could be modified to so that both pistons operate braking circuits? This would also involve shifting the assembly to lie central with the brake operating arm, leaving a bit more room to fit a separate clutch MC.

Steve
Steve Gyles

Steve,

Are the bores of the clutch and brake sections of the Master Cylinder the same?
(Too busy to research it).


Mick
Mick Anderson

Steve,

I like your thinking, but you've still got the balance bar problem, although possibly with less space or width needed.

I'm approaching it from the twin cam perspective, so don't have the combined brake/clutch anyway. Easy enough to get one though.

More thought is needed.
Surely someone who races has dual circuit.

Here's another variation I took at Silverstone this year.

http://tinyurl.com/of84a

Look at all the space there if we shortened the pushrods and brought the whole thing forwards.

Significant safety improvements, even for standard road cars too.

Mark.

Mark Hester

The one flaw with the MGA MC idea, is that as Steve himself found out, when one side went (in his case the clutch), the "unaffected" brake circuit failed too. I thought that a dual brake system would have to have more redundancy than that. That would imply that a separator wall needs to be built into the reservoir to completely split the two sides. This then means that you have a truly dual system

As a keen diver, I can use the analogy of a twin tank setup. If the two tanks are connected (using a manifold usually) it means that you do not have a truly dual / redundant system, because if something on one tank fails, until you turn it off using the valves on the manifold, the other side will just bleed across as well. You effectively just have a very big single setup with a big reservoir.

In my circles the use of completely separated dual tank setups is the norm (ie two separate tanks, each equipped with its own regulator(s) and gauge etc, and the dual safety is effected by the "rule of thirds" (breathe one third of tank A capacity, switch regulators, breath one third of tank B, switch and breathe 1/3 of the volume left in A, switch to B etc ad infinitum / 0 bar). At any stage in the dive, if a system fails (regulator freeze, O ring blow, hose failure - they all happen), you have enough air to continue or abort and return to the surface (with deco stops) on the surviving tank. This was taught to me by some seriously advanced cave divers, and works very well for deepish (65m) deco diving in the English Channel and in the South Pacific too.

What about Steve's idea of using the standard MC, adding a divided reservoir in some way, and then switching the clutch to cable operation (many cars these days seem to have a cable operated clutch)?
dominic clancy

I have an MGB dual circuit system apart right now along with the MGA system, brackets and all. I'm in the process of experimenting with the geometry, however, I don't like the look of the plastic MGB MS. I think the way to go is with the 3 seperate girling etc. master cyls and balance bar.

Mick and Mark, I don't see the problem with the balance bar that you are concerned about. Almost all race cars use them.

Fred H

Dominic,

The clutch cable idea is interesting. Certainly solves the space issue.

Later dual circuit masters come with a brake fail light. Plus your not introducing the problem of balancing the two master as with two separate m/c's.

Fred,
Since you have all the parts out.
How much further forward in mm's would you have to move the cage with the MGB Plastic m/c (yes I know they are ugly) fitted to the MGA cage?

Cheers
Mark.
Mark Hester

I took some pics of the 2 setups side by side, but I can't post them. Shoot me an email and I'll send them to you.

I'm not sure what you're asking, but the MGB ms and bracket needs to come down about 1.25" (which could easilly be done by cutting and welding) and about .5" forward(which might be done with the pushrod).
Fred H

Mark,

The brake failure light can be used with any type of dual brake system, the tandem master cylinder or the side by side master cylinder with crossbar.
The failure light operates when there is a pressure difference in the front and rear brake lines.

Mick
Mick Anderson

A pictures worth a 1000 words as they say.

http://tinyurl.com/hs3lo

Certainly from other views I have the MGB cage would need to be lowered to clear and MGA bonnet.

It seems that with a modified MGB cage dual circuit brakes could be easily installed...

Much safer for everyone and still possibly to return to standard and signs of the mod being done..

Your thoughts ?

Mark.
Mark Hester

Master Cylinder Bore Sizes

MGB (Non Servo) = 3/4
mga twin cam = 7/8
Pushrod MGA = 7/8

Thus the MGB one will not offer enough pressure.
Thanks Mick for emailing me these details.

A Wilwood 7/8 Tandem or Dual Master is 207mm long

http://www.wilwood.com/Products/006-MasterCylinders/009-ATCMC/index.asp
or
http://tinyurl.com/q7soe


Fred, Could you post the lengths of the MGA & MGB Masters please.


Master Cylinder Lengths:

MGB (Non Servo) = ??? mm
MGA Twin Cam = ??? mm
Pushrod MGA = ??? mm
Wilwood 7/8 = 207 mm

Thanks
Mark.
Mark Hester

Mark

Actually the MGB M/C will give a higher pressure for the same pressure by your foot on the pedal. Smaller piston diameter = smaller area = higher pressure to counteract the same pedal force. What is does NOT give you is the same volume for a given stroke. In fact the MGB M/C will give you just under 75% of the volume as the MGA M/C for a give stroke.

The important parameter becomes the volume required by the braking system actuation by the M/C piston. IF you have drum brakes on all wheels I would assume that the (stroke)volume requirement is higher than that for a disc/drum system such as the B uses. For those of us with discs on the front it would appear logical that the B M/C would work since the disc/drum setup is very similar. Only difference will be the diameter of the rear slave cylinder pistons.

Does anyone know what the A and early B rear slave piston diameters are?

Larry


Larry Hallanger

Larry

Following your logic that drums all round require more volume to be displaced than a disc/drum combo per pedal push, why did they increase the capacity of the master cylinder when discs were introduced?

Steve
Steve Gyles

For a drum brake, as the shoe wears, the brake shoes get adjusted by changing the distance between the cylinder and the shoe via the eccentric. Over the course of time the total volume required in the wheel cylinder to compensate for this wear stays constant. That is because all of the adjustment is done external to the hydraulic system.

In the disk brake, as the pads wear they must be pushed closer and closer to the disk. This requires increasing volumes of fluid in the caliper and, as such, less and less fluid in the master cylinder. All the adjustment is done within the hydraulic system. Whether that extra volume in the M/C was actually required when the 1600 disk brake system was implemented, I don't know. Maybe it was a carryover from the Dunlop brakes used in the T/C?

Incidently, by monitoring the fluid level visually in your late model car, you are actually monitoring the wear of the pads. If you have ever added fluid before changing pads, then, replaced the pads, you may find a puddle on the ground from the excess fluid pushed back to the M/C.

Chuck
Chuck Schaefer

Opps. I just remembered that the T/C used different master cylinders. All the other stuff applies though.

Chuck
Chuck Schaefer

All,

I have done the wilwood conversion. I found with all the parts it was still less expensive than a new orig type m/c. I haven't messed with the balance bar a whole lot. I have found that the pedal feel on the brakes is the best I've ever felt on an MGA. The clutch is another story, with the 3/4" bore of the Girling M/C and the increased leverage of the wilwood pedal assy, there is NO feeling to the clutch, but it all works reliably despite all of the "race only" warnings on the packaging.

If anyone is interested I have the part number for the pedal assy. I believe there are about 3 different versions and it took me a little time to figure out which one to order.

Here is a pic of the install.

http://i109.photobucket.com/albums/n54/johnb_09/MGA1/wilwoodmcs.jpg

j
JohnB

This sums up basically what I'm on about.

http://tinyurl.com/ovj4l

As you can see to fit an MGB or any Dual Master Cylinder for safety you need to cut a hole on the firewall or do as the above owner has done.

MGB is 3/4 and not 7/8 so there will be more pressure exerted at the wheel cylinders and also more pedal movement/travel required.

Fred, any luck measuring the lengths of those masters yet?

Cheers
Mark.
Mark Hester

This thread was discussed between 01/08/2006 and 14/08/2006

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