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MG MGB Technical - LHD Brake Master Cylinder

Does anyone know what the split is on the US spec brake master cylinder as used up to Dec '67?

Thanks

Colin
Colin Parkinson

I don't understand what you want to know.

FRM
Fletcher R Millmore

I thought that US cars had single circuit brakes until 1968? My '67 is US-spec and has single circuit.
Dave O'Neill 2

Right, single circuit to Rdst 138400, GT to 139471, when '68 model year production started 10/67. US regs, Moss numbers.

FRM
Fletcher R Millmore

'68 models and up had the dual circuit braking system. RAY
rjm RAY


Hi guys, I have some duff info from the SC Parts catalogue! I do not have the Moss US spec catalogue, so cannot give you the part number, but it is the dual circuit cylinder that is used without a servo.
What I want to know is the % split for the front to rear circuits.

Colin
Colin Parkinson

Aah found it! Moss US part number is 180-765.


Colin
Colin Parkinson

Colin -
Both are the same - no split, just tandem pistons, so two circuits with the same pressure.

FRM
Fletcher R Millmore

FRM, so the split front to rear is 50 / 50. That is unusual for a dual circuit master cylinder. Would'nt that mean that the 2 bore sizes were both the same?

Colin
Colin Parkinson

Colin -
I don't think I've actually met a single odd split cylinder - they all have a straight bore. But then I don't work on any new stuff. Exception is some "quick fill" ones that have a larger bore which fills the slack space with fluid, then the main bores take over, but the mains, and consequently circuit pressures, are the same.
There's a cross section in the shop manual.

FRM
Fletcher R Millmore

The front/rear balance is achieved by using rear cylinders that have %20 of the braking force used on the front. RAY
rjm RAY

Ray, what I am trying to do is to get less on the rear on a race car, by using possibly a master cylinder with a 60/40 or 75/25 split.
I know I can reduce braking to the rear with different slave cylinders with the drum brakes.
But another experiment is to use rear discs. But I would have to find a caliper with a very small piston.

FRM I do not have a factory manual or indeed any manual which shows the US spec master cylinder.

Thanks for your help.

Colin
Colin Parkinson

Colin -
Sorry I don't have a scanner, somebody must.
Do you actually know of cylinders with such splits? Seems to me that it would require quite an unnecessarily long cylinder to be fail safe.
Reducing the diameter of the rear disc would greatly reduce effective braking.
Grease the pads! Sorry.

FRM
Fletcher R Millmore

FRM, yes I do know of cylinders like this. In fact all the dual master cylinders I have intimate knowledge of have a split differential. Unfortunately most of them bolt direct to a servo unit. I do have a couple of Escort / Capri cylinders which I may be able to overhaul, and modify to fit.
The other option would be to use the MGB dual circuit cylinder, and to put a proportioning valve in the rear line.

Colin
Colin Parkinson

Colin
Why cant you just remove say 25% of the rear shoes which will reduce the brake efficancy?

Ste
Ste brown

Colin -
Interesting indeed. I should like to find some links to info on this. All cylinders I've looked at are straight bore except for some with the initial filling function. I don't see a reasonable way to actually get a split from the cylinder proper. I do know of some that have proportioning valves built in to the cylinder (Escorts post about 93 for one, at least for US cars). This may be more common on newer cars than on stuff I'm familiar with. Perhaps these are what you speak of? I've lost interest in new cars & parts with the advent of ABS!

I've not had one of these integral valve cylinders apart, but imagine you could diddle the prop valve to give what you want. You could not do so if the split were somehow derived from the basic cylinder. It's on my junkyard scrounge list for when (IF!) the weather ever improves.

You need to be careful about "proportioning valves", separate or integral - the term is used wrongly, even in factory books and other "official" things, to include simple limiting valves, not the same thing at all. Descriptions of braking systems can be misleading. Split ratios of the system as a whole are not reflective of actual master cylinder output ratios, even if these do have a split produced by integral proportioning valves. And stated design splits may vary considerably over braking range; you never know where in the range the quoted figure applies.

The limiting case is very good brakes and rubber where both rear wheels are off the ground - the actual braking split is then 100-0%. Just before that you have 99-1, and ideally you do not want the rear locked then, but a 75-25 system will have been locked for a long time. Presumably you are operating in this range on a decent race car. About now you may come to the same conclusion I have - antilock brakes are a very good idea - on rear brakes only! It works very well on my 89 Dodge pickup, the first implementation of antilock things I'm familiar with.

Brake system redesign is a valid place to be very picky about such things, I think!

FRM
Fletcher R Millmore

Aftermarket adjustable proportioning valves are a common item on racing cars. Usually, they are mounted inside of the car, within the driver's reach, so that the brakes can be fine tuned while underway. A friend of mine installed rear discs on a Suzuki Samauri and an adjustable proportioning valve cured his problem of early lockup of the rear brakes. RAY
rjm RAY

And going back to an early question as to why the split master had equal pressure in both circuits: If it didn't the rear cylinders and/or calipers would need changing for the new master in order to restore the brake balance.
P Hunt

Just out of interest, I have in 1 of my boxes of bits, a master cylinder for the Mk2 Escort. This has a tag on it showing 60/40.
So I wasnt dreaming!

Any way with the B, where I am going to play with rear discs, I will use the standard type cylinder, and a proportioning valve to turn off braking to the rear.

Colin
Colin Parkinson

Colin -
Can you get a picture of that cylinder? I expect it has a prop valve built in, and that's what the tag refers to. Is the tag a semi permanent part of the cylinder or a paper throwaway? I'd look it up but I need more info - what year is the car? Is it a Mazda based Escort? And of course, I may not be able to find it if it was not US supplied or had a different name.

FRM
Fletcher R Millmore

FRM

This is the Ford Escort Mk2

http://en.wikipedia.org/wiki/Ford_Escort_(Europe)#Ford_Escort_Mark_II_.281975.E2.80.931980.29

Definitely no Mazda input ;o)

You'll probably need to C&P the link
Dave O'Neill 2

Dave -
Yes, I thought as much, US Escort were mostly completely different. Never got any good stuff until the 91-95, when it was really a Mazda Protege LX (323 sedan w/BP 1.8 DOHC engine and 4 wheel discs in the rest of the world) with different sheetmetal. At least the GT was, the others got Ford engines instead of Mazda, and blew up regularly. These used Mazda brakes complete, but about 93 they started using a Ford MC with built in prop valve. Mazda stayed with plain MC and separate valves.
Anyway, the point of all this is that I think you want a plain MC if you are going to add a valve, or a MC that you know the split of, and ideally can change at will. No idea what two prop valves working at cross purposes might do!

FRM
Fletcher R Millmore

Dave -
Yes, I thought as much, US Escort were mostly completely different. Never got any good stuff until the 91-95, when it was really a Mazda Protege LX (323 sedan w/BP 1.8 DOHC engine and 4 wheel discs in the rest of the world) with different sheetmetal. At least the GT was, the others got Ford engines instead of Mazda, and blew up regularly. These used Mazda brakes complete, but about 93 they started using a Ford MC with built in prop valve. Mazda stayed with plain MC and separate valves.
Anyway, the point of all this is that I think you want a plain MC if you are going to add a valve, or a MC that you know the split of, and ideally can change at will. No idea what two prop valves working at cross purposes might do!

FRM
Fletcher R Millmore

"a master cylinder for the Mk2 Escort. This has a tag on it showing 60/40" Completely different marque, irrelevent as far as the MGB is concerned. Mini single circuit systems with drums both ends had a proportioning valve for the rears to reduce the pressure there, but again irrelevant as far as MGBs are concerned.
P Hunt

Actually Mr Hunt (sorry I do not know your first name), it is relevant in the context of what I am doing. I may well use this Escort cylinder, to get some of the bias off the rear.
This is a track car! And as you may know, when a B is pedalled hard, there is too much bias on the rear.

Colin
Colin Parkinson

I still think that the after market, adjustable proportioning valve is the way to go. They're inexpensive and easy to install wherever you want so it can be easily adjusted to suit the conditions at hand. Most of the units that I have seen are mounted inside of the car within easy reach of the driver. They work very well. Good luck with whatever you use. RAY
rjm RAY

Fletcher, there should be a pic of the Escort cylinder below!

Colin


Colin Parkinson

Colin -
Thanks, but I need the other side! If it has a prop valve in there will be bulges in the side and probably some visible machining that does not match a plain cylinder. I think I can see a, or the, bulge, just.

FRM
Fletcher R Millmore

This thread was discussed between 24/02/2010 and 08/03/2010

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