Rear Brake Slaves

[PeterTR7V8]

Hi all. To upgrade the rear drum brakes do I get a bigger slave or a smaller slave cylinder?

It might be a stupid question but I keep thinking if the cylinder is bigger then the piston will move a shorter distance per millilitre of fluid that enters the cylinder. After that I get a bit cloudy on what my name so I can't follow that thru with a conclusion.

Do I just need the slaves off a SD1 or do I need a bigger master cylinder?

I don't want to do a rear disc conversion thank you.

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[jclay (RIP 2018)]

If you have a 5-speed car, you want the slaves from a 4-speed set-up. The are a smaller diameter and therefore will move further with the regular master cylinder.

jclay

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[PeterTR7V8]

Jclay, I don't know what the car was originally. It's a 1977 model, it used to be a 2L & it currently has a 3.9 diff but I don't know if the diff is original.

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[FI Spyder]

Why do you want to upgrade the rear brakes? They will lock up before the from discs do if they are working properly. Assuming all pivot points are free and greased (brake grease) auto adjusts are working, and the rear hoses are open (if they are old, replace 'em as they get old they constrict internally.)

TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra

[jclay (RIP 2018)]

All five speed differentials were 3.90 until mid run in 1979 when they when to 3.45. If you use the four speed slaves it will increase the speed of the cylinders moving with the same movement of the master cylinder.

It would probably be better, easier and cheaper to get a modified proportional valve from Ted even with the shipping.

jclay

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[PeterTR7V8]

Cheers for the info.

My pv is sitting on a shelf having been replaced with a willwood on the rear circuit a few months ago. Are the SD1 slaves smaller?

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[Underdog]

I assume you want to increase the rear brake force. I don't know the sizes of the wheel cylinders/slaves but you want ones with a larger bore. Fella on Woodys site races and claims MGB ones are larger. Not sure which he was refering to as the GTs have a larger bore than the roadsters. He states that the stock cylinders are too small to get near an equal bias. This was done at the factory so there would be no chance of rear wheel lockup.
Same amount of pressure acting on a larger surface area (the piston) equals more force. Simple hydraulics rule.

72 MGB BRG
80 TR8 Persian Aqua
If women don't find you handsome, they should at least find you handy.

[busheytrader]

Hi Peter,

I fitted the 4 speed rear brake cylinders to my 7V8 but only after I'd uprated the front brakes.

The 4 speed rear cylinders provide more pressure to the shoes than the 5 speed items. However, the 4 speed shoes are narrower so both standard setups provide roughly the same braking effort. Fitting the 4 speed cylinders inside the 5 speed drums will increase the rear braking effort a touch.

I did this as I have more powerful brakes up front instead of the weedy, easily faded, set fire to them twice, lock up all too easily in the wet standard setup. I wouldn't uprate the rear brakes unless the front were uprated first.

It's an easy mod and has worked for me for almost 20 years. A small hole has to be drilled in the 5 speed backplates to accept the 4 speed cylinders. A mounting pin is in a different location.

Adam

TR7 V8 DHC Jaguar Solent Blue. 9.35cr Range Rover V8, Holley 390cfm, JWR Dual Port, 214 Cam, Lumention, Tubular Manifolds, Single Pipe Exh, 3.08 Rear, 200lb Spax & Solid Bushes, Anti- Dive, Granada Vented Discs & Calipers, Braided Hoses, Green Stuff, 4 Speed Rear Cylinders, Uprated Master Cylinder & Servo, AT 5 Spokes and Cruise Lights. No Door Stickers. Mine since July 1986, V8 from 1991

[PeterTR7V8]

Conflicting info!

The fronts are uprated alright & yes I want more braking at the rear wheels because I want to be able to dial out the rear lock up with the willwood valve but at the moment they are unable to lock up.

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[Underdog]

Peter, as I said, I don't have specs on the different wheel cylinders. As I said though, for what you want to do, you need to look for slave/wheel cylinders with a larger bore. I have a bias valve to install myself and uprated frt brakes so I may well be looking myself. I took a quick look at the catalogs but they only quote part #s not specs. If the 4spd ones truely are of smaller diameter, I don't know why anyone would install them.

Don't feel bad about the conflicting opinions. This subject came up on the MG site and was debated for at least 3 pages!!

Brakes are a hydraulic system and the same rules apply. Some get confused by volume of fluid and movement of the pistons. The system is completely filled with fluid which doesn't compress. The shoes & pads are adjusted so that minimal movement is required to apply braking force. So the small variance in the size of a wheel cylinder will have negligible effect on pedal travel.

Think about earth moving equipment or a log splitter. The rams have huge bores to develop tons of force. The same amount of pounds per square inch applied over a greatter area equals more force. Now in those extreme cases, volume & speed of travel do come into play hence the need for a hydraulic pump.

I had personal experiance with this a few years back. The parts store sold me the wrong rear cylinders for my old Chevy wagon. Evidently two brake options were offered. The ones I got were larger bore. You could feel the back end squat down and at first I thought the additional braking was great. That was until the snow flew and it would lock the rears on slippery roads. I swaped back to the correct ones.

I'll see if I can gain any info on sizes and what will fit. I'm interested myself.

72 MGB BRG
80 TR8 Persian Aqua
If women don't find you handsome, they should at least find you handy.

[Wayne S]

As an aside, are TR8 wheel cylinders different to TR7 4 and 5 speed ones?


<b>2.0 Litre DHC Grinnall
Red 4.0 Litre V8 DHC Grinnall (with huuuuuge arches...!)</b>

[Underdog]

Interesting question. I never gave it a thought, assuming all the 5spd axles got the same. I just looked at my TRF catalog which I believe to be a reprint of the factory catalog. Guess what.. too different wheel cylinders for Tr7 & TR8 5spds. And get this, the part # quoted for TR8 is the same as the #(GWC 1213) for a 4spd TR7. So according to that, if you have a TR8, installing 4spd cylinders would be no difference since the factory already did it. Still wish we had some specifications rather than part numbers to compare.

72 MGB BRG
80 TR8 Persian Aqua
If women don't find you handsome, they should at least find you handy.

[jclay (RIP 2018)]

TR8s had upgraded brakes in the front. Looks like busheytrader is correct.

jclay

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[Mowog73]

Underdog is correct, a larger wheel cylinder gives more braking power. As an example, the MGBGT using a bigger rear wheel cyliner than a B to give it more rear braking power. It was used because the GT is a heaver car than a B.

If the 4 spd wheel cylinder is smaller than a 5 speed's then swapping a 5 for a 4 speed wheel cylinder will give you less braking from the rear wheels.

Mark

<font size="1">1973 MGBGT Teal Blue
1976 TR7 Mimosa
1980 Spitfire 1500 Vermillion</font id="size1">

[Cobber]

<font size="2"><font face="Comic Sans MS">Here goes!
In your mind's eye, try to think of the differences in hydraulic cylinder size as comparable to gear ratios.
A slave cyl with a larger dia will need more pedal travel to move a given distance but with more pushing power.
A slave cyl with a smaller dia will need less pedal travel to move a given distance but at a cost to pushing power.

So you can compare the larger dia cyl with a low gear ratio (produces a lot of torque but at low speed) where the input shaft turns more turns times relative to one turn of the output shaft than the high gear ratio (produces high speed low torque)which can be compared to the smaller dia cyl.

Now I hear you ask why did they fit the smaller cyls to the TR8,s?
As I haven't seen a real TR8 so I can't say for sure but I assume the callipers are larger than those of the TR7, so I'd say they did it to reduce the pedal travel which would be longer with the TR8's larger callipers! Another factor may be to do with the differing weight balance If the TR8 has more weight over the front axle, then under hard braking there would be less weight over the rear axle than the Tr7 there by causing rear wheel lock up!

I hope this explanation is a bit clearer than <b><font size="4"><font color="brown">Mud!</font id="brown"></font id="size4"></b></font id="Comic Sans MS"></font id="size2">

80'Triumph TR7, , 73'Land Rover (Ford 351. V8),
'89 Ford Fairlane
85'Alfa 90, 69'Ford F250.
76' Ford F100