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Late GT6 wheel cylinders


Roger K
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Please would somebody let me know the definitive and correct wheel cylinder size for the rear brakes on a non-roto GT6, later than KE20000?

I've got a pair from Rimmer's sold as for the late type, but they are 5/8".  Another website has indicated they should be 7/8"....

Anybody know for sure?  And does anyone know where I can get a pair, made by a reputable maker (Lockheed, Delphi, etc.) rather than County - had some bad experiences with County cylinders on my Land Rover...

Thanks,

Roger

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Ah! Ha! Having earlier mistaken a thread about GT6 clutch master cylinders for brake master cylinders I came out with all the brake hydraulic upgrades.  :wub:

However, if we're talking brake hydraulics, the late GT6 rear brake cylinders, KE20,001 onward are 7/8"

Rimmers and Canley don't have them, but last year found some on Paddocks and bought a set, just in case.

Doug

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OK, it looks like I need 7/8" without the slotted casing, to take the auto-adjust - GWC1501.  No such beast any more, so it'll have to be the 3/4".  I'd guess the bigger bore cylinder (that's now NLA) was to match up to the servo'd discs up front in the last of the line.

Spoken to everyone who might know, and there is no stock anywhere of these and no chance of them being remanufactured, unfortunately.  If Powertrack have no insight, I don't think anyone else is likely to...  the best advice is next size down, which is GWC1117 for MGC etc. which is a 3/4".  These are only available in the cheaper castings, but I suppose beggars can't be choosers.

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Roger,

Yes, sadly Paddocks don't stock 7/8" anymore although 3/4" are about and described as Triumph parts. I think you're right about the configuration between servo and larger rear cylinders and the servo only working on the front. On previous models where the servo when fitted was as an optional extra, it worked on all 4 wheels. I've tried this on mine and very quickly put it back to front only! It made braking very peculiar, much too much on the back end. Which makes me think if you fit 3/4" there might be some benefit to extending the servo to the back.

Also the 7/8" repair kits are still available. 

James Paddock Limited - Triumph Stag, Spitfire, TR6, TR7, GT6, Vitesse, Herald Parts and Spares Specialists. WHEEL CYLINDER REPAIR KIT LATE MK3 SELF ADJUST(UKC1261)

Doug

 

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I've had a few searches this morning; the 7/8 wheel cylinders were also used on Ford Corsair and Lotus Elite but it seems none are available bar repair kits; I've linked to a couple of other sellers:

https://shop.oakmeremotorgroup.co.uk/product/rear-wheel-cylinder

https://www.sjsportscars.com/parts-and-accessories.htm#searchresults

Do you want to keep the auto adjusters, Roger? Might be easier to bin them and go for the earlier type.

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I'm just renovating the entire rear end, and the cylinders I removed are 5/8" unfortunately, so not correct.

I think all the alternative makes that used these are using smaller sizes now so you can't trust the Lotus or Ford sellers as their websites will just say 'suitable for' or something similar.

I'm trying to keep the car as original as poss so will go with 3/4" for now in the hope that either something comes along, or a more expensive marque that used them gets a batch cast.  Happy to pay proper money, just like to get things right.

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Ah...

The next question is, are the manual adjuster cylinders available in 7/8"?  - and can they be adjusted with the drums on?  I'm guessing from your post, Doug, that they're not.... I can't see how you could adjust brake shoes accurately without a snail cam arrangement, accessible with the drum on.  Tighten up, back off two clicks...

edit - I see, need to change the lot - backplates, shoes, drums, cylinders and adjusters.  The problem then is that I'm restricted to 1.25" shoes instead of 1.5", and still only 3/4" piston diameter.  Quite apart from the drop in rear braking power this would give against the front servo'd system, I don't know if the extra 0.25" would create clearance problems with the drum fitment on the hub against the backplate, or the longer driveshafts etc. - so it could turn into an expensive experiment.  I'll leave it as it is for now, with the 3/4" auto-adjust cylinders, and see how I go.

Edited by Roger K
Did more research!
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To confirm previous postings, coincidentally I received a pair of late late GT6 MkIII rear brake cylinders from James Paddock this morning, JP part number JPG400 (2xGWC1121).  These are 0.75" internal diameter (3/4") and have the correct end fitting for the auto adjuster, they are unmarked as to manufacturer and unpackaged.

My car (late swing spring MKIII) has identical cylinders fitted so non standard.  They have been on the car since probably 2009 or possibly before (fitted by PO) and the braking has always seemed OK, if anything the rear wheels lock before the front if provoked.

There are several postings on other topics covering drilling access holes in the brake drums to allow the auto adjuster cog to be turned by a small screwdriver to adjust the brakes with the drum on.  Note that the adjuster arm needs to be pushed or pulled away from the cog to allow it to rotate freely for adjustment.  My adjusters are in pretty good condition and hopefully can be made working with new handbrake springs and judicious filing.

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Thanks Chris, I have sourced some 3/4" cylinders to use so should be OK.

With regard to size, given that a set amount of fluid is pumped to the rear brakes when the pedal is pressed, I would expect smaller pistons to move further for the same brake application - so it would make sense that your 3/4" cylinders might lock up the rear brakes early, which (if excessive) could definitely be a safety issue.  Hence the factory's fitment of 7/8", to effectively lessen the rear braking effect.  I think fitting the largest option currently available is the best solution.  Some suppliers are sending 5/8" cylinders out for late GT6s, which could make the brake balance worse.

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39 minutes ago, Roger K said:

given that a set amount of fluid is pumped to the rear brakes when the pedal is pressed

That's a case of "you can prove anything if you make up the first step". It's not true. When you press the pedal, you generate pressure, and the fluid moves to where that pressure moves things. As a result, all the parts of the braking system get the same pressure. If you have a bigger piston, the pressure creates more force and it pushes harder on the shoes. Meanwhile, the servo input has that same pressure and has the same effect on the front circuit. So your conclusion is exactly wrong because your fundamental assumption is exactly wrong.

The factory fitted 7/8" rear cylinders to increase the rear brake effort, to match the increased front brake effort achieved by the servo.

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59 minutes ago, NonMember said:

That's a case of "you can prove anything if you make up the first step". It's not true. When you press the pedal, you generate pressure, and the fluid moves to where that pressure moves things. As a result, all the parts of the braking system get the same pressure. If you have a bigger piston, the pressure creates more force and it pushes harder on the shoes. Meanwhile, the servo input has that same pressure and has the same effect on the front circuit. So your conclusion is exactly wrong because your fundamental assumption is exactly wrong.

The factory fitted 7/8" rear cylinders to increase the rear brake effort, to match the increased front brake effort achieved by the servo.

So what you're saying is: the piston increased in size, but the fluid chambers are the same? 

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1 hour ago, NonMember said:

The factory fitted 7/8" rear cylinders to increase the rear brake effort, to match the increased front brake effort achieved by the servo.

Very interesting, and confusing !:wacko:

If that's the case why didn't the leave the 3/4" on the back and have the servo on all 4 wheels as they had done on the previous model?

My own jaundiced view is that they were discontinuing the line and had a load of bits in the stores they wanted rid off, so they messed about until they found what worked.

Doug

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1 hour ago, Colin Lindsay said:

So what you're saying is: the piston increased in size, but the fluid chambers are the same? 

No, what I'm saying is that it's never the volume of fluid that matters. Bigger slave cylinder bores mean more "gain" in the hydraulic system, so your foot goes down further but you get more braking for the same pedal "effort".

1 hour ago, dougbgt6 said:

If that's the case why didn't the leave the 3/4" on the back and have the servo on all 4 wheels as they had done on the previous model?

My own jaundiced view is that they were discontinuing the line and had a load of bits in the stores they wanted rid off, so they messed about until they found what worked.

Not impossible. Did the change to bigger cylinders (and servo on front only) come with the wider drums?

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14 hours ago, NonMember said:

No, what I'm saying is that it's never the volume of fluid that matters. Bigger slave cylinder bores mean more "gain" in the hydraulic system, so your foot goes down further but you get more braking for the same pedal "effort".

That's where my confusion comes from. Roger says:

"I would expect smaller pistons to move further for the same brake application" which seems to make sense - if you pump a pedal and fluid moves then a smaller bore will take less fluid than a larger one, so will move further or for less effort.

If your foot goes down further, surely you're making more effort at the pedal, or moving more fluid? That extra movement must be for a reason; the fluid does not compress so it must be greater volume of fluid being moved?

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47 minutes ago, Colin Lindsay said:

If your foot goes down further, surely you're making more effort at the pedal

No, exactly NOT. If you have bigger slave pistons, then your foot does go further but with LESS effort. As Clive has just posted - more force on the shoes for less force on the pedal.

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