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MG MGB Technical - US spec Master cylinder

Can somebody just confirm which port goes to the front and rear on the US spec dual circuit master cylinder please?

Thanks

Colin
Colin Parkinson

If you mean the later boosted dual circuit master the front circuit nearest the driver, rear circuit nearest the front of the car, as per the attached. However that is for an LHD, on an RHD the front circuit pipes are reversed as the drivers side always goes down to the caliper that side and the passengers goes over the heater.

Other sections in manuals are confusing at best if not actually incorrect.


paulh4

As it's US-spec, I would imagine it's this un-boosted one, although I can't answer your question.

Does it really matter?


Dave O'Neill 2

I took this photo while building my GTV8 using the later style in-line servo and dual circuit cylinder. Hope it helps.


Mike Howlett

US had both types at various times, the non-boosted had a separate manifold and pressure balance switch with the pipes leading to the corners of the car. Moss.com has a drawing indicating that it is the other way round i.e. in both cases the ports closest to the driver is for the rear circuit and the ones closest to the front of the car are for the fronts. If you don't want a pressure balance fail warning you don't have to fit it, just plumb the front and rear pipes direct to the master.
paulh4

Dave you are correct it is the US spec un-boosted cylinder.

Yes, in my opinion it does matter. As you press the push rod it moves the first piston which pumps fluid to that brake line. That piston then pushes the second piston which pushes fluid out to that line.
But there is a spring between those pistons which in my opinion would mean the first line would get fluid before the second. So I think the front brakes will come on marginally before the rears if the first port goes to the front brakes.

Am I correct?

If not can some explain why?

I have an answer from an American forum and he assures me that his car has the pipes the other way round. I think that is wrong.

Colin
Colin Parkinson

The boosted master makes sense having the front i.e. primary circuit closest to the driver and hence the push-rod and first piston for the reasons you say. So why Moss show them the other way round for the unboosted I don't know.

However in normal use the space between the two pistons is filled with fluid so as soon as the first circuit starts to pressurise the second piston will start to move, the spring only gets fully compressed when the first circuit has failed with no pressure.

But the second circuit takes a little more pedal movement to start pressurising after the first circuit has pressurised because of the arrangement of the bypass holes from the reservoir, I've labelled these 'A' and 'B' in the attached.

'A' feeds fluid into the first circuit and to the space between the two pistons where the spring is, and 'B' into the second circuit. But main cup '12' has to get past hole 'A' before it starts to pressurise the first circuit, and only then will it push on the second piston. The second piston has to move main cup '4' past 'B' before the second circuit starts to pressurise. This does compress the spring a very small amount as well as resulting in the second circuit pressurising slightly after the first circuit.

The fact that the WSM labels the first piston cup '12' and the second '4', without labelling either of the sections as 'primary' or 'secondary', could be taken to mean that the second circuit i.e. the delayed one is the primary for the front brakes, as Moss shows. As I say I have no idea why and it doesn't make sense ... unless it was guarding against cup 15 failing which would drain the first circuit and if the front brakes were off that you would lose the vast majority of your braking effort. In which case, they obviously decided that didn't matter with the later boosted master.

Having said all that, Haynes DOES label the pistons 'primary' and 'secondary', but the other way round implying the front brakes should go to the first circuit i.e. nearest the push-rod, as in the second image.






paulh4

Hmmm--close, but not quite
I think a lot of people including Moss get mixed up because this cylinder sits 180 deg around compared to the later booster cylinders
I'll try to explain how it works-----

The piston closest to the pushrod/mounting flange end of the cylinder is the front brake circuit, the piston further into the cylinder is the rear brake
When the peal is depressed the main pressure cups on both pistons clear their reservoir orifices at the same time, the spring between the pistons is strong enough to make both pistons travel as one at this stage--The primary/front piston relies on the opposing cup on the second/rear piston to allow it to build pressure in the front circuit--
So, as the pistons move down the bore as one,the rear brakes piston builds up enough(very minor) hydraulic pressure against the end of the cylinder to restrict the movement of that piston allowing the front brake piston to compress the spring so that the front brake piston can then build pressure between it's cup and the second cup on the rear brake piston.
IF the rear brakes fail, then when the brakes are applied both pistons will travel down the bore, but because there isn't any rear brake pressure then both pistons travel as one until the rear brake piston bottoms out against the end of the cylinder--only them is it possible for the front brake piston to travel further, compressing the spring and allowing the front brake piston cup to pressurize against the second cup on the rear brake piston---giving front brakes only at a lowered pedal height
IF the front brakes fail both pistons initially move as one , as the rear brake circuit starts to build pressure the front brake piston overcomes the strength of the spring and the end of the front piston bottoms out on the rear brake piston producing a solid mechanical piston to operate the rear brakes only but at a reduced pedal height.

Think that covers it
willy
William Revit

Yes, about to correct myself with that very fact, if I'd looked a bit further in my notes I would have found it.

The bottom line is that for both types of master the primary circuit for the front brakes comes off closest to the pushrod, and the rears off the secondary which is furthest away.

You should get effective pressure on both circuits at the same time when both are working, and the same long pedal if either has failed. I think.
paulh4

yep
William Revit

yep
and with the later master cylinder with the built in bias valve, if one circuit fails the piston in the bias valve moves over and blocks off the failed circuit so that a higher/fuller brake pedal is retained even though one circuit has failed, but with reduced braking power and a brake fail light illuminated
William Revit

I'd not come across that, makes sense, thanks.

I do know that to recentre that shuttle one only has to partially unscrew the switch, unlike the earlier system where you had to bleed first one circuit then the other perhaps more than once to centralise it.
paulh4

Thanks Willy and Paul. I am convinced I have piped it up wrong.

So I will take it all apart and have another go.

Dont know where I got the info that made me do it wrong in the first place.

Colin
Colin Parkinson

Yeah, it's really strange, theoretically you would think that both circuits would have the same or close to the same pressures and it would work with the plumbing reversed--but it doesn't for some weird reason and the brakes just don't feel right.
And, if I could offer some advice/warning---if you're retrofitting one of these cylinders (or any dual circuit cylinder for that matter) to a car that had a different cylinder, it's important to make sure that there's enough pedal stroke to get full travel of the master cylinder or very close to it--I know a bloke that fitted one to his Magnette as a safety thing, While it was all bled up and everything it was fine but when he cracked a rear brake pipe he lost all his brakes and couldn't work out why- On inspection we found that the cylinder was only getting about 2/3 of it's stroke so loosing one end left the pedal almost touching the floor before the other end started to work at all--

willy
William Revit

Colin
What is the year and serial nbr of your US car. There were changes and upgrades during the years. I'm looking at two repair manuals, Chilton and Haynes.

Cheers

Gary
79 MGB
Gary Hansen

Gary, it is a modified 1971 race car.

I have answers to my questions now about the pipes.

colin
Colin Parkinson

OK.. Thanx. Glad you got the item sorted

Cheers

Gary
79 MGB
Gary Hansen

This thread was discussed between 24/03/2021 and 28/03/2021

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