Master Seal Kits (single circuit systems)

The following was developed from an investigation into clutch master repair kits, but the same considerations apply to brake masters as well, with even less info from supplier sources.

The repair kits and other internal components differ between the various masters - some using a cup-type main seal and others a ring-type. The factory changed from cup-type to ring-type "sometime in 1973" (Clausager) but bear in mind a later replacement could have been fitted to an earlier car as they were interchangeable. This image (Brown & Gammons) appears to identify which clutch kit goes with which master by saying the 'new' assembly uses the ring seal, but this Moss Europe page indicates that GMC1007 (i.e. the newest type and current replacement) uses the same kit as AHH6553 i.e. the original master which is obviously incorrect.

The drawings are not very helpful. My 73 has no grooves at the flange, my 75 (as shown below) has one, not the two depicted by Moss Europe (Moss US refers to "two concentric circles at end or grooved by flange" which is more like it).

The remanufactured clutch master with the metal reservoir doesn't seem to have any external markings going by photos, leading one to buy an early kit unless you look at the innards first.

The confusion continues: For the clutch, Moss Europe for example shows drawings of the cup-type pressure seal in item 3, and a ring-type pressure seal in item 4 (both types use a ring-type seal for the secondary seal). But click on those items and both kits show two ring-type seals, with item 3 coming up as GRK3007 with seals, dust cover, piston, shims and spring which is a conversion kit from the earlier cup seal to the later ring seal at 12. Item 4 comes up as GRK3004Z with just the seals and dust cover, and is a repair kit for those masters with the ring-type piston at 5. However the Moss detail states it is for master AHH6553 which is the Mk1 master, which takes cup seals. That was replaced on Mk2 cars by BHH4667, which I think is angled over slightly to clear the larger North American dual brake master and cap. The Parts catalogue says that was replaced "sometime in 1973" (Clausager) by AAU7152 having 'two concentric rings'.

Brown and Gammons show GRK3007 for the early master with a cup seal, a ring seal and the dust cover, and GRK3004 for the later master with two ring-type seals and dust cover, plus the brake master restrictor. So ostensibly the same part numbers as Moss, but the early one is NOT a conversion kit to the later type of seal.

Both of the GRK3004 kits also show a restrictor valve (the white item) that is only used in brake masters, as this kit is apparently correct for both the later clutch and brake masters using ring-type seals - the implication being that they are the same bore. But the restrictor must not be used in the clutch master or it will delay clutch engagement. So if you have the 'early' master you may well think it only needs the cheaper kit with the cup seal, but if someone has already fitted the other kit with the ring seal, then you need the ring seal kit as for the later master! If replacing the seals on the early type, and its history is unknown, you will need to remove the existing internals and check which piston you have. If it has the cup-type piston then from Moss you will need the more expensive conversion kit, although cheaper cup seal kits are available from other vendors. If you Google the two part numbers you will get both types displayed for each, but if you need the cup-type you will need to confirm with the vendor that is they one they are going to supply, and not the ring type.

However if you have the cup-type seal I strongly recommend that you use the Moss conversion kit. The cup-type seal uses a plastic spreader which isn't supplied as part of the kits, and isn't available from anywhere else. Bee's broke and caused the seal to tilt to one side, and almost certainly caused the intermittent loss of pressure. The ring-type doesn't use that and can't suffer from that problem, maybe why it was changed. Fortunately I had a good spreader from one of the previous master replacements.

Leacy shows the GRK3007 cup seal-only kit on the purchase screen, but at 12 which is the price of the Moss conversion kit! As I said at the beginning the situation is very confused.

For the V8 (same item as the Midget 1500) the Parts Catalogue only shows one clutch master cylinder BHA5217, and one repair kit 8G8730, which many suppliers show with the cup-type seal. However having replaced the clutch master on the V8 with an OE type that shows the groove by the mounting flange, the same as the replaced brake master. Brown & Gammons shows just the one repair kit GRK3005 with a cup-type seal, I've not looked in mine to see what type is there. They also show GMC1011 as the current item with the plastic reservoir and a 17.8mm dia piston, one would have thought that has the ring-type seal, although several suppliers show the same repair kit for the Range Rover Classic and Discovery 1 with the cup-type seal, still no spreader.

For the brake master for all MGBs i.e. including the V8 there doesn't seem to be a conversion kit. Moss Europe show just one seal kit GRK1026 for the early type with the cup-seal which they describe as 'original m/cylinder', and another GRK3004Z for the later and after-market types with the ring-seal i.e. the same as for the clutch above which they describe as 'aftermarket m/cylinder'. This is confusing - by 'aftermarket m/cylinder' they could mean the plastic reservoir type, but usually a suffix letter after the original part number indicates that it is a substitute from another manufacturer. Both kits contain the 'slow-return' restrictor.

Brown and Gammons show GRK1026 for master cylinder GMC122, with the cup-shaped seal and restrictor, with a leaflet headed 'Plastic type clutch master cylinder and slave cylinder replacement', but only contains parts for the master! They also show GRK3004 for master cylinder GMC1007, with two ring-type seals. This replaces GRK1009 for master cylinder GMC150. All the kits show the restrictor.

So they both specify GRK1026 with cup seal, but Moss says it is for the original master cylinder, and B&G show it with a leaflet that says it is for the 'plastic type' master cylinder. Did I say it was confusing?

With MGOC you are on your own as since they changed the web site they don't tell you (or me at any rate) what anything is for.

The shim (arrowed) that fits between the seal and the piston with cup-type seals. Note! If you are fitting a new dust cover to the push-rod do so from the piston end before refitting to the master cylinder, the sharp edges and large size of the clevis pin fork can rip the seal.

Bee's broken clutch seal spreader that came out (bottom) and a good one from an old cylinder (top):

The spreader does not appear to be available meaning if this is broken or lost the ring-seal conversion kit would have to be fitted.

The distorted seal. The vertical line runs through the centre of the pip that is at the bottom of the cup and sits in the hole in the spreader. That should be central, but is clearly displaced to the right because of the broken spreader.

A selection of pistons and seals: A is the new piston from the conversion kit, B an identical item from a previously changed cylinder. C is from the V8 clutch and has two parallel rings! D is probably from the roadster brake master and identical to the one from the clutch master.

The cup-type seal removed from Bee's clutch (A) significantly longer than a similar seal from another cylinder (B). If the replacement seal is like B, that would explain why the biting point is now lower. (It wasn't, that turned out to be a problem with the release arm and bearing alignment with the cover plate).

Piston and cup-type seal from Bee's clutch (A) significantly longer than the piston and ring-type seal of the conversion kit:

Three from Richard Massey on the MGOC forum - top from a 100 mile no-name plastic-bodied replacement that started leaking, middle from an older TRW plastic, bottom from a metal can. Slight differences in thickness of the flanges either side of the seals which is irrelevant as long as the seals are in the same position. Also slight differences in diameter of the flanges but only 3 thou max between any of them. Speculation that it was deliberate to give greater clearance to the bore, but I can't see that after all these decades, and it would allow for more lateral movement of the piston in the bore which could impact the sealing. Whilst my old pistons do show some polishing on the sides of the flanges there was no scratching or scoring in the bore which the 100 miler shows. What (to me) is more interesting is the long extension inside the spring of the no-name plastic. It shows how much free space there is at the bottom of the cylinder, and I can see it being needed in some applications to prevent the piston going too far and tangling up the spring, which then may not push the piston back as it should when the pedal is released. Neither do you want to push the secondary seal past the inlet port which will cause fluid to leak out.