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MG MGB Technical - Brake Master Cylinder

I've a '72 B with single line brakes. The master cylinder (original metal type) has a slight weep behind the pushrod boot. I thought a re-seal might be in order, but understand there are two types. Apparently one has concentric ring markings at either end, but with m/c in situ I can't see these. I've a couple of spare cylinders, but these externally look a bit ropey.

My question is are the internals interchangeable between the two types, if one had the correct piston? Or put another way are the bodies identical, but just the seals & piston different (acknowledging that you don't get new piston with kit). I'd like to change the seals with the m/c in situ, and odds on I've got a spare piston of the correct type, so my thinking is if I bought the wrong kit I could make up a correct one with all the bits. just trying to minimise downtime.

Hope all that makes sense.
Peter Allen

It's a mess. For a start the suppliers talk about the concentric rings being either at the flange or at the outlet, but what they show at the flange are parallel rings, and my 75 only has one ring on both the clutch and brake masters.

Whilst Moss has a conversion kit for the clutch including piston Brown & Gammons uses the same part number for just the later seals. I'm not aware of a similar kit for the brake.

Moss show a cup-seal kit GRK1026 for what they describe as OE and a ring-type GRK3004Z for 'aftermarket', which might mean the plastic reservoir type, or the Z might just mean the kit is aftermarket not the master!

B&G show a cup-type GMC122 and a ring-type GRK3004.

Really you are better off looking to see what you have now. Firstly even though it's a 72 the master may already have been changed for a later type as they are interchangeable, and secondly the bore may be corroded. I've changed three of my four masters in the past, but on finding some at least with good bores when the roadster clutch pressure seal failed I had a look first and the bore was perfect.

More info here http://www.mgb-stuff.org.uk/masterseal.htm primarily on clutch kits but also showing the various pistons.
paulh4

Thanks, Paul.

I stripped down my two spare brake m/cs and also a spare clutch m/c. They all had what looked like identical pistons and cup type seals. The brake ones with the slow return restrictor shown in your comprehensive attachment. Internally all three spares looked superficially good, so I think I'll gamble on the cup seal kit on basis that if it's wrong for one in car, it'll be at the least useful to overhaul a spare one. I was just wondering whether with these changes they'd changed the body of the cylinder.....if they didn't, then if I've the ring type in the car, I can convert to the cup type with the spares I've got that aren't in the kit. Otherwise, 'Vehicle Off Road' for a couple days, VOR as they used to say in my local British Leyland dealership!
Peter Allen

Bore sizes is something I've not compared, and it is possible the conversion kit with the piston contains ring-seals of a slightly different size to those in the later ring-seal only kits. The only size information I can see on the two sites is Moss saying GRC1026 i.e. cup-type seal is 0.75" bore.
paulh4

All the bores, both brake and itch, were 3/4" until the tandem brake master cylinder.
Not at base so might have a memory wobble, but my recollection is that the version with a ring primary seal was a change by Lockheed either late in the production run or shortly after.
Identification as I recall is by rings machined on the outside of the cylinder. Though I always have a look inside anyway.
Innards are interchangeable as an assembly, including the spring.
Paul Walbran

That is 4 cylinder cars of course, V8 had different clutch hydraulics.
Paul Walbran

Thank you Pauls, both, and great, Paul W, on the interchangeability point. That's exactly what I needed to know.
Peter Allen

Peter, one thing you had better check if swapping is the overall length of the piston and primary seal, to confirm the seal is in the right position relative to the entry port from the reservoir. Measure from the outer end of the piston to the lip of the primary seal. Just on case.
Paul Walbran

According to Clausager the change was made 'sometime in 1973' and whereas my September 72 built 1973 model does not have markings my 75 V8 does as attached.

Moss Europe and Brown & Gammons are confusing regarding identification as they refer to 'concentric rings', and Moss shows two rings near the flange end. These would be parallel rings, not concentric, but in fact there is only one of them as in the attached. What would be concentric is the alternative identification of one small ring inside another on the side of the cylinder near the outlet, but I've not found those on mine.

The point about the conversion kit is that it can be installed in the earlier master, and henceforth you would need the later ring seal repair kit if you didn't have an old-style piston. Hence you really need to look inside anyway, even on the later master in case someone has used on old piston and done a reverse conversion!

The V8 did have different bores for both master and slave. Otherwise the same considerations apply, although B&G is only showing the earlier cup-type seal kit and Moss isn't showing anything I can see.



paulh4

To close this one off, I recently removed piston assembly from in situ master cylinder and discovered I had ring type. As said, I'd bought the cup type seal kit already, so as I had spare cup type internals from another master cylinder, I swapped all these into ring type m/c. Pistons and springs are very different in length between the two types. My glamorous assistant helped me bleed brakes and I have a good pedal. Haven't had a chance to drive as yet, as doing other stuff whilst apart, such as replace pedal bushes etc. Hopefully all will be well.
Peter Allen

Give Peter a call at North West Automotive Hydraulics, 0161 7992653.
What he doesn't know about Lockheed/Girling isn't worth knowing and he can supply the right bits.
Allan Reeling

Thanks, Allan. It was more an exercise in seeing if the bits would swap from one type of master cylinder to another, as the repair kits are pretty affordable (if fragile: the new dust seal split getting it over the fork on the pushrod).
Peter Allen

Looks like it would be easier from the other end, before the push-rod is fitted to the master. Another time, that is.
paulh4

I did think that (subsequently!), Paul. The rubber (if that's what it is) didn't feel terribly elastic.
Peter Allen

This thread was discussed between 22/05/2019 and 20/06/2019

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