GT brake master cylinder

Tips and help requests for your cars mechanics. points gaps, timming settings all those sorts of things

Re: GT brake master cylinder

Postby Orrp01 » Sat Aug 11, 2018 3:14 pm

I messaged the guy on eBay several days ago asking if it had internals but he hasn’t replied. My guess is that it has nothing inside and it is a bit expensive for the outer shell only. The securing ring on mine is only damaged on the side ie a few small holes on the side which is well below where it seals. I have it steeping in molasses atm to clean the rust off. I’ll see what it’s like after cleaned but reckon it should be ok. If the one on eBay was complete I would probably buy it but I think the seller is trying to be clever not mentioning or showing it to be complete.
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Re: GT brake master cylinder

Postby thomas » Sat Aug 11, 2018 4:10 pm

Cheers, Orrp01 let us know how it works out, it has been an interesting discussion.

Supplies both of SP2381 kits and complete master-cylinders look like becoming critical. The problem with these retaining collars/rings adds to the agony, increases demand and must lead to many otherwise good master-cylinders becoming scrap. These rings however look like something someone moderately skilled, in possession of a clue, and with access to a biggish chucked lathe, could knock out a few out of in their lunch-break, from solid alloy, as long as the dimensions were near enough and they could replicate the coarse internal thread; they don't have to be a works of art -it might be worth trying any general engineering works near you to see if they could oblige, you might be pleasantly surprised.

I noticed one or another of the sp2381 kits I spotted recently on ebay might not have been complete, there should be five parts in the genuine Girling ones, they are: pushrod valve seal; cylinder seal; seal for reservoir extension joint; seal for inside of cap; spring washer.

:goodluck:
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Re: GT brake master cylinder

Postby jpsmit » Sun Aug 12, 2018 3:04 am

coming back to the Spitfire - I know the pistons are the same as I used them to rebuild mine. Likewise if you look at the other MC's on Moss Motor's website are 7/8" bore - (22.25mm)

OTOH at the brakeparts.co.uk website it says the bore for an HB MC is 20.6 mm - so, I think <think> that means that you get a little more pushing power with the larger bore. I do know that when I replaced the MC on my Midget it came with a smaller reservoir - this seems to be the way of the world. Certainly a closed system shouldn't need too big a reservoir - and wear will be slow.

Finally, I am aware of a company or two in the USA who will resleeve MC's with stainless - people seem very happy with the results.

and, of course there is the aftermarket companies - wilwood etc.
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Re: GT brake master cylinder

Postby thomas » Sun Aug 12, 2018 10:12 am

I would think this 'TR6' one on the same site looks a better tentative match for upgrading a single-circuit disk-braked, servo-ed HB to dual-circuit:

https://mossmotors.com/brake-master-cyl ... ftermarket

I would of course need to know the diameter and the depth of the boss on the end, and the distance between the mounting-hole hole-centres, to either rule that or any given master cylinder out, or consider it a 'hmm, possible'. Data like this should be available up-front with their products, but no doubt a query should elicit some useful info. It doesn't mention the bore at all, but it's reasonable to assume it should be adequate. Two circuits means twice the input effort required and the bore I expect would be on the largish side as the effective piston travel is halved.

Taking into consideration the masking and force multiplying effect of the servo in-between, by pretending its not there:
A smaller cylinder bore will give a lighter pedal, but it would have to travel further to build the same pressure as a larger bore one would, and might run out of travel before effective braking is applied, or at the other extreme be so light as to easily lock-up the wheels; a bigger bore will give a harder pedal, pushing back more, a wooden-feel if unassisted, and would take up slack and build pressure and thus activate sooner.

Trial and error, and finding what other people are doing/thinking and learning new stuff is the only way to find what fits and works well and will be sustainable into the near-future. It's no good someone saying they got their's sorted with something that is equally as rare and exotic as the original-equipment kit is becoming or will one day become. Aftermarket options - and with that increased safety margins, compatibility with other classic models assuring longevity and availability and continuing spares support - definitely looks the most promising route in the medium term when keeping the OE stuff serviceable could be grimmer still. I've no desire to pioneer in this area, at the moment brakes are immaterial as another bout of welding/bodywork/paint is in progress and last I recall the brakes were good to excellent, and spares are readily to hand, but take an interest in the subject with an eye to the future.
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Re: GT brake master cylinder

Postby jpsmit » Sun Aug 12, 2018 2:08 pm

Again, not an engineer but the bore on a dual circuit is the same - there are two seals in series so, I don't believe there is any difference in pressure - or negligible at most.
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Re: GT brake master cylinder

Postby Orrp01 » Sun Aug 12, 2018 7:47 pm

That looks like a good replacement and I’ve found some info on bore size. Apparently it is .812” for one circuit ( same as viva) and .77” for other circuit. Looking at a picture of it on a tr servo unit it looks very similar. I think I’ll measure the distance between mounting holes on mine and collar dimensions and make a phone call to some of the sellers. Being dual circuit it would be a much safer system and to me would take precedence over my 48 year old original lol. These seem to be readily available and quite cheap but will have to find out if reducing the pressure to the rear brakes would have much effect. Will post here if I find out more info.
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Re: GT brake master cylinder

Postby 1972nail » Sun Aug 12, 2018 9:49 pm

I know this company are out of stock but there may be some useful info here or available by email. https://brakeparts.co.uk/shop/Vauxhall/ ... g%20Valves
Editor of the VVOC Magazine ReVival

"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.
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Re: GT brake master cylinder

Postby thomas » Mon Aug 13, 2018 7:23 am

1972nail wrote:I know this company are out of stock but there may be some useful info here or available by email. https://brakeparts.co.uk/shop/Vauxhall/ ... g%20Valves


That's an excellent find, single

https://brakeparts.co.uk/shop/Vauxhall/ ... es/BMC4688

or better still tandem

https://brakeparts.co.uk/shop/Vauxhall/ ... es/BMC4689

and service kits for the same. Thank you.

Can't see pictures yet -'image coming soon', but bookmarked and noted, against the day.
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