pedal effort required in dual circuit versus single brakes

[chicagojeff]

Hi all:
I have an early S3 coupe (late 66) that has been fitted at some point with the 0.70" dual circuit brake master cylinder, and no servo. Odd, because Super Safety would have been 0.75 with a servo, so this may be some previous owner job. At any rate, what I am trying to determine is why I seem to require so much more pedal effort than my friends' cars with the same 0.70" bore single circuit. Ideas?

I thought for sure someone improperly installed the .75" larger bore, and that when I removed it I would find that confirming serial number. The larger diameter and no servo would explain the extra effort required. But, guess what? serial number 61676597 is the right 0.70" bore as far as I can tell.

Any thoughts on the relative effort needed? Pads are all stock. Thanks much, Jeff in Now Moderate Climate Chicago

[UAB807F]

Hi Jeff,

Last year I converted my S3 to a 0.7 dual m/cylinder and removed the servo, so I have a similar system and like you I was concerned about pedal pressures. It's fine in practice, so if you feel yours is excessive I think the first thing I'd do is check that the pistons/calipers are moving freely and there's not something sticking. Next off would be the pad material itself just in case the previous owner had fitted pads more suited to track days, etc. Especially if the car did have a servo at one point and still retains the old pad material because that was different to non-servo cars.

Everyone has their own favourite pad material/maker so all I'll say is that I've had both standard and Greenstuff pads in the front calipers, the latter do have a higher friction and more grab so it feels like the brakes are working better with relatively low effort. There are downsides, more dust and some folks have quoted poor working life but mine have been ok so far.

Brian

[lotuselanman]

Gidday,
What Dual M/C did you use? From what car does it originate? I am interested in fitting a Dual M/C to my +2S.
Les.

[UAB807F]

Hi Les,

It was from the Triumph Spitfire 1500 & late GT6 range. There are snags to this choice, firstly on the Triumphs it is mounted on a sloping bulkhead and so the reservoir isn't suited to a vertical bulkhead mount. Not a problem in my case as I wanted to fit a remote reservoir with a low fluid warning light anyway, but it would be if you wanted to retain the original looks. Secondly the later units are metric threads, again no problem but I didn't realise that at the time & it slowed down my installation.

I bought mine very cheaply from a Triumph enthusiast for ?45 last year although there was another ?25 or so on the remote reservoir (Land Rover I think ?). It all fits in quite well, here is a link to the set-up in my car. I think others on the forum have used units from Westfield with an integral reservoir which does fit beneath the carbs, so there are plenty of options.

http://www.martley.plus.com/2011/uab10.html

Brian

[elanmac]

I recently fitted a Westfield .70 master cylinder (ebay ?26) Factory ?128.I use silicone brake fluid and greenstuff pads. I have noticed no difference in pedal pressure. It has a built in low fluid level switch.
Mac

[Bud English]

Brian - Thanks for the heads up on the metric fittings. I knew I had to change the reservoir but didn't see that coming. I have the M/C all mounted up in my rebuilt pedal box. I never checked the fittings. You've saved me from a last minute headache. It will be a bit before I'm ready to mount the box.

Thanks

[Gray]

Hi

Classicar Automotive still list the Elan dual circuit cylinders.

I used their cylinder and adjustable pushrod on mine, not on the road yet so no feedback on effort.

Regards Gray

[Galwaylotus]

The pressure produced by a master cylinder of a given bore, e.g. 0.70 inches, is the same whether single or tandem configuration. P=F/A. For a given force at the calipers, the force applied to the m/c is fixed and is simply the ratio between the m/c and the caliper piston bores squared (ratio of the respective areas).

[chicagojeff]

dear all: fitted Greenstuff pads tonight, and bled the lines. I tried the more "gentle" pedal pressure to bleed the 0.70" bore dual circuit MC without pushing through to the second piston chamber. Seems pretty firm. Now to bed in the Greenstuff pads and see if it made much difference than with Mintex pads. will report back.

[chicagojeff]

Results: bled the system, using the workshop's recommendation that with each bleed one does not press the brake pedal all the way down. Road tested the Greenstuff pads. Actually seems pretty good. Can chirp the tires with a hard press at 40 mph, so seems all right. thanks for everyone's input. Jeff

[Tahoe]

Brian and Bud,

What remote reservoir did you use with the triumph master?

[UAB807F]

I bought it from Ebay and seem to recall it was listed as a Land Rover part. I paid ?20+postage for it, and it looks exactly like this one at Carbuilder Solutions at ?38.

http://www.cbsonline.co.uk/product/2_Chamber_Brake_Fluid_Reservoir_FLRES2

It has a low level switch and 2 chambers. I made my own mounting bracket and it fits quite neatly (as in "no extra room" ! ) on the inner wing alongside the airbox. The spacing of the outlets match those on the m/cyl, so you can mount it underneath the webers. I did try that option but it's just as awkward as the standard one to fill, so it went on the wing. If you are mounting it remotely you can use almost any reservoir, even a single chamber providing it's split into 2 lines at the outlet because in the event of one circuit leaking and draining the chamber there should still be enough in the lines to operate the secondary circuit.

Brian

[Galwaylotus]

With a remote reservoir mounted in the engine bay just under the bonnet I have no need for a level sensor. A quick glance each time the bonnet is open tells me if the level has dropped.

[Bud English]

I'm still in the body off stage so I haven't sourced one yet. I'm holding off until I know where I'm going to mount it.

[Galwaylotus]

My installation. AP master cylinder and AP remote reservoir. No level sensor required!!