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MG Midget and Sprite Technical - Frogeye rear brakes

Moving onto the rear brakes now. Given that I have fitted disc front brakes on my Frogeye project, and I need to buy new rear cylinders, should I be fitting something other than the standard GWC1114 rear cylinders?

I have the standard backplates so cylinders will need to be of the single piston, sliding type. Standard ones are 7/8" bore, but maybe there are other sized ones available, if an alternative would be advantageous?
GuyW

Guy

What sized master cylinder are you running?

One school of thought is that if you are running 3/4" inch dual master cylinder (and 1275 engine and therefore clutch) them use a Morris Minor 1000 (I.e. 1098cc engined later Minor) rear wheel cylinder GWC1116 which will fit the Frogeye backplate but is 3/4" inch bore.

Others will be along shortly and say do not bother changing the rear cylinder or the master cylinder.

NB different pushrod lengths for 7/8" drum braked master cylinder compared to the 3/4" disc brakes late MkII and (some? All?) MkIII Sprites. Again some will say - does not matter.

I am in favour of using 3/4 inch brake master cylinder and rear wheel cylinders when upgrading MKI and MkII Sprites to front disc brakes. I.e matching the bores of the later spec cars. You can swap out the rear backplates for later ones if you want to use the later dual piston rear wheel cylinders (3/4" bore).

There are a few archive threads on this.

Useful background info:
http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm
&
http://www.gerardsgarage.com/Garage/Tech/DBrake/TandemMC_Part1e.htm

If you want a dual 3/4" master cylinder of good quality (AP Caparo) then Powertrack Brakes were able to source them recently. Again others will tell you this is an unnecessary change.

Cheers
Mike

PS I am happy with front disc brakes combined with 3/4" rear wheel cylinders (Morris 1000) and a dual master cylinder with 3/4" brake master cylinder bore and correct pushrod for that bore in my 948cc former front drum braked MkII Sprite (7/8" bore retained for 948 gearbox clutch slave cylinder)

M Wood

Thanks Mike,
I haven't decide on the mastercylinder yet. I am aware that there are those who argue in favour of either alternative.

I was thinking more of balance between front and rear. With more powerful front brakes I wondered if there was an alternative rear cylinder that would increase them to maintain the same front/rear ballance. So that would be the 3/4" GWC1116
GuyW

Guy, if you decide to use the floating rear cylinders (GWC1114) I have some new, unused ones together with shoes that I will not now be using as I now have backplates from a later axle. Let me know if interested.
Bill Bretherton

Guy

I am convert to the GWC1116 Cylinders and 3/4 bore master with discs. I used to have the GWC1114 cylinders but found they upset the balance and were too powerful for the rear and it locked up too easily. The change rectified it. (thanks for the advice Alan Anstead)
Bob Beaumont

Pleased to have been of assistance - Bob.
Alan
Alan Anstead

Thanks to Alan too for the same helpful advice to me a while back on using MM 1000 rear wheel cylinder (GWC1116).

Best wishes
Mike
M Wood

The balance is dependand on cylinder diameter brake construction and friction material: I Use mintex 1144, standaard front calipers and the mkIII rear brakes. At first with dark soft material now with hard linings. Tires are much beter than in 1958 so you have to shift the balance more to the front.
Flip Brühl

I converted my master cylinder using brass tube from a model engineering supplier. Easy to find the right size that fits the original 7/8" bore and brings it down to 3/4". Sealed in with good quality silicone and it won't go rusty! The tricky but is drilling the ports and then de-burring them so they don't ruin the seals.

I already had one piston and made one to match on the lathe (needed 2 as I have a 1275 clutch and slave). The double ended rear slaves are much better than the original ones with the handbrake cam on top of the piston. With ceramic pads and standard solid discs on the front the brake balance is excellent.

Les
L B Rose

If you use 7/8" diameter rear cylinders, I would expect them to lock the rears very easily.

As already said, the Frogeye rear cylinder is the same as a Morris Minor, the rear cylinder from the later 1098 engined car is 3/4" diameter, and will improve the brake balance.

I would still test to make sure, as we have the smaller rear cylinders on our Minor, and without a pressure limiting valve in the rear circuit, the rears lock under heavy braking, just when you really do not want them to!

The static front/rear balance is something like 64/36% with the 7/8" rear cylinders and 71/29% with the 3/4".

The master cylinder diameter has no effect on brake balance, only on the pedal pressure required to make them work. Assuming no other changes, the smaller the master cylinder diameter the lighter the pedal pressure, BUT the more the pedal travel.

Again assuming no other changes, the difference in pedal pressure between a 7/8" bore master cylinder and a 3/4" one is very significant - about a 25% reduction using 3/4".

Richard
Richard Wale

Richard, this makes sense to me, and as I said earlier correct sizing of the rear cylinders is about front rear balance, rather than anything to do with selection of the m/c (which is a different issue).

What I am not clear about is that if disc brakes at the front are more powerful than the original 7" TLS, why is it that one needs to REDUCE the rear braking effect, and not INCREASE it? Surely if the front brakes more effectively, then the rear should be increased by a similar amount in order to keep the same front / rear balance.

Addendum:
Now tell me that it is about the change in the rate of weight transfer. With more effective front brakes, the rear goes lighter so the rear wheels are more prone to locking. Is that it?
GuyW

My standard disk brakes did not lock earlier than the standard drum brakes. And I can easily lock front and rears with my 7/8 M/C . The disks don't fade after 2 stops after heavy braking wich the drums did . Even with the standard drum brakes with the same linings front and rear, the rear locks first (passenger side if there is no passenger). Tires are much better that is the only reason there is more weight transfer to the front.
Flip Brühl

Guy,

The standard Sprite/midget discs are not 'more powerful' as such, the pedal pressures and brake balance are almost identical between the standard Frogeye 7" front drums, and retrofitting the front disc setup.

The main reason for the rear lockup is the 7/8" rear cylinders - the bore is just far too big!! They create a massive front/rear braking imbalance. The problem was addressed on the 1098 engine Minors and the disc-braked Sprite/Midgets by reducing the rear cylinder bores to 3/4". On the Minor, the front brakes were also increased at the same time to 8", so the front/rear balance was very significantly moved to the front (74/26% from 63/37%).

Yes, and compounded by discs that do not fade very easily and much grippier tyres, so can put more braking force into the fronts, and unload the rears.

Richard

P.S. The quoted pedal pressures, front/rear balance % etc. all come from an Excel brake design model that I created, based on the design parameters directly from AP Lockheed. If anyone would like a copy please let me have an email address and I will send it to you.
Richard Wale

Guy,

Don't forget to make sure that you remove the 'residual pressure' valve at the bottom of the brake bore in the master cylinder, if you are planning to use the drum braked version - not fitted to the disc braked master cylinder.

Please see items 28 and 29 in the attached file for details.

Richard


Richard Wale

Richard,

What problems would occur if the residual pressure valve was not removed? I fitted discs to the front of my frogeye with drum brake MC as it came and wasn't aware of any problems created.
David Billington

David, the disc brake pistons don't release from the disc as they should, and you will get a period of brake dragging after each use. You may not notice with gentle driving, but it can cause overheating.
GuyW

Guy,

Thanks for that but I'm not aware of suffering that even with hard driving and the drum brake MC. I did at times get some brakes locking on but that was years after fitting the correct disc brake MC as IIRC the drum brake MC wasn't available at the time, I put that problem down to the disc having worn thin and causing issues.
David Billington

It is a real problem as our local tyre fitting operator will vouch for first hand of not removing the residual pressure valve when fitting disc brakes!

I completely missed removing it when fitting discs to our Minor, but had noticed a scratching sound after a few miles of running.

A while later I took the car to have the front wheels balanced, the fitter and I both noticed that the front wheel was hot, but did not expect the first wheel nut to burn his fingers when taking it off.

Not serious, fortunately, but a very good reminder to remove the valve (if fitted).

Richard
Richard Wale

Pictures of the two types of master cylinder valves at: http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm

You can get a spare disc braked one by breaking a scrap 3/4 inch master cylinder (also gives you the correct pushrod)

Cheers
Mike
M Wood

I know that I have commented on this before and don't wish to labour the point but.....

I have a 7/8 master cylinder with residual pressure valve still fitted and disc brakes. No overheating, no drag, no excessive pad wear. None of this stuff fits with my experience and I can't help feeling that there is a lot of "over-thinking" going on here.

I have had 7/8 rear cylinders and don't think I ever got them to lock up before the fronts. I now have 3/4s fitted and can't detect any significant difference in braking performance on my road-going frog.
Simon
Simon Wood

obv it will depend quite a lot on how they are driven. Also with a bit of play / wear in the FWBs there will be consequent knock-back of the pads thus circumventing the problem.
David Smith

I guess a bit of run out or warping of the discs would also obviate the problem.

What is it about the original early drum braked wheel cylinders that requires a restrictor valve in the m/c? Rear cylinders on later cars work perfectly well without a restrictor in the system.
GuyW

Guy, I presume you meant "residual" valve. In the case of drums I presume the valve is mainly assisting the front drums as they apply more braking effort than the rears. With later disc/ drum configurations I presume it isn't so important to hold the rear shoes on for a short while as the front pads are doing most of the work. However, the pads need to be released promptly so that accounts for the change to a restrictor valve.
Bill Bretherton

I should have added that the first incarnation of disc brakes on the Minor was Midget hubs, discs and calipers, and the smaller 3/4" rear cylinders - so some relevance to this topic!

In the Minor Workshop Manual there is an explanation of what the residual pressure valve does, but it does not appear in any of the official BMC Sprite or Midget manuals:

'The combination inlet and outlet check valve in the head of the cylinder is provided to allow the passage of fluid under pressure from the master piston into the pipe-lines, and controls its return into the cylinder, so that a small pressure of approximately 8 lb./sq. in. (·56 kg./cm.2) is maintained in the pipe­lines to ensure that the cups of the wheel cylinders are kept expanded; it also prevents fluid pumped out from the cylinder when 'bleeding' from returning to the cylinder, thus ensuring a fresh charge being delivered at each stroke of the pedal.'

The reference to 'bleeding' is relevant, as the Minor now is a bit of a pig to do, although much helped by using a Sealey Pressure Brake Bleeder.

Richard
Richard Wale

Yes Bill, quite right I did mean to say Residual prssure valves.

I can see the point of maitaining a slight pressure to keep the rubber of the cylinder piston seals expanded on the drum braked cars. The 8psi retained would do that adequately wihout being sufficient to prevent the pull-off springs from easing the brake shoes off the drum surface.

Whereas, if used on a disc brake car then that 8psi pressure is transferred direct from the pads to the disc as there is no proper mechanism for pulling the pads clear. That said, 8psi is very little for a braking system so with freely moving pads I can see why it might not always cause hot brakes! I don't know what pressures are attained in these cars, but modern servo systems readily exert 400 lbs / sq inch.
GuyW

Guy,

With no servo and 100lb (45kg) pedal pressure the brake line pressure is around 700 lb/sq in with a 7/8" m/c and almost 1,000 lb/sq in with a 3/4" m/c.

With a servo up to double that, and a bit more, depending on the ratio.

Richard
Richard Wale

That, Richard, is a mighty lot! Rather more than I thought.

For my car, I have what I assume is the original 7/8" m/c. Pretty rusty althgough as far as I can tell the bores look OK. The imternals won't readily remove, other than the pushrods and the two pistons. Beyond that the springs and whatever lies beyond are clinging fiercely at'yon end.

I then also have what is a refurbished (or new?)3/4" m/c powder coated in black with a red cover plate. It has pushods but when I removed the end plate there are no innards at all - just two sausages of oil soaked tissue paper. I don't know if it was bought by the PO without pistons and the other internal parts, or if they were wrapped seperately and have parted company along the way, - I don't recall ever seeing anything like that amongst the boxes that were this car.
GuyW

Guy,

They are the calculated pressures, but very similar to what is found typically in most cars.

Richard
Richard Wale

OK, I went for the GWC1116 rear cylinders (3/4") and they are now fitted. (sorry Bill!)

Fitting was a close run thing as I had already installed the rear hubs and bearings and torqued the big nuts up nice and tight.
Only then did I find that the cylinders wouldn't quite clear the hub flange. Fortunately, slackening off the 4 backplate fastening bolts gave enough movement of the backplate and sufficient clearance, - just, to wriggle the cylinders into place.

Researching photos of numerous 1/4 eliptic cars I have confirmed the route for the brake pipes, and where they are strapped to the axle just inboard of the suspension brackets. I was a bit surprised that this arrangement gives enough flexibility for the cylinder to slide up and down freely without stressing the brake pipe from continual flexing. But I guess its ok.
GuyW

I'm looking forward to putting mine in, Guy. They came out with the axle fully assembled, but it was one of those jobs where it won't come that way, nor this way, nor the other way, then suddenly it's free. Of course I wasn't able to make a note of the method, because success was unexpected.

Life's like that, isn't it . . . . . . . ?
Nick and Cherry Scoop

Guy
The original Lockheed rear cylinders just miss the flange. The latest modern variety in my experience is just a shade bigger externally.

As regards the brake pipe, I used to think the same about fracturing but many years of frogeye and morris minor ownership has indicated its not a problem!!
Bob Beaumont

Thanks Bob, for that reassurance. I had been browsing photos on the internet and a couple I came across had been modified with extra flexi-hoses fitted from beside the axle casing strap to the wheel cylinder. Seems a bit unnecessary!

Any sources, these days, for 3/4" m/c internals - especially the pistons? I read up on the Gerards garage site that Mike liked me too. They had some specials made, but as far as I can tell that was a limited run back in 2007
GuyW

I might have internals for a 3/4 MC. The bore was a bit pitted on mine so I replaced the whole cylinder
Bob Beaumont

Talking of work hardening, has anybody still got the helical approach to the clutch slave - or is it standard practice now to switch to a length of flexible?
Nick and Cherry Scoop

Guy, I have no problem with you using GWC1116's! I can see it's a good option for you. As I have the later backplates I'll get the later Spridget 3/4 cylinders I think even though the axle won't then be as original. I do wonder if the sliding cylinders have a tendency to seize?
Bill Bretherton

Bob - that would be brilliant, if you can / wish to spare them! If you can locate them can you contact me "off BBS" on

>firstnameATwellerHYPHENlakesDOTcoDOTuk>
(use the symbol characters in place of the capitals)
GuyW

Guy

Another source for 3/4" dual master cylinder pistons might be Coastings Ltd (or perhaps they may just do rebuilds). New 3/4" AP Caparo master cylinders were and may still be available from Powertrack Brakes. Not sure what Past Parts can supply, but maybe worth checking.

You will still need the correct pushrods, just from a secondhand disc braked pedal box from a later MkII Sprite and an early MkIII Sprite (late 1962 to 1964 or 1965?) (these do come up for sale - got one recently with a master cylinder for rebuild for £50, including the correct pushrods),

Another option is using the Sebring Sprite pedal box with separate master cylinders.

Cheers
Mike
M Wood

Aha Mike - that was from me! Glad it went to a good home, I rescued what was left of a rusty wreck from a farm outside High Wycombe - they were going to scrap the lot, sacrilege given how rare some of these parts are becoming.
David Smith

Thanks David - great saving of bits!

Cheers
Mike
M Wood


The thought has just occured to me (!) that, if using the later Spridget back plates and cylinders, would one use the later handbrake mechanism (across the axle) or retain the Frogeye one? I'm not at the assembly stage so can't try it out yet.
Bill Bretherton

Bill,
The swivelling Christmas tree handbrake mechanism remained the same, post Frogeye, up until the 1500. After that the method changed and is more efficient.

The only other change l can think of was the earlier frogs used a longer handbrake lever. I'm not sure why it was changed. Maybe it was a 'borrowed' component from some other car?
GuyW

Thanks Guy. I've now read the Moss catalogue properly and it seems there was an update but the older rods are NCA so later ones are recommended anyway.
Bill Bretherton

There must have been another change then. Maybe the length of the rods was altered.

The rods I have - which I assume are the originals, both have a small split pin braised to them, part way along each length. I have yet to work out why!
GuyW

The 'Christmas tree' lasted until 1977. It was only the last few years of production that had the updated - and efficient - cable.
Dave O'Neill 2

Dave/ Guy, Moss give an earlier part number for up to earlier HAN6 and GAN1 models (rods only) so there may have been slight modification, but the RH rod is NCA anyway. I have Frogeye and 1275 model rods so I'll compare them when I dig them out.
Bill Bretherton

When I converted my Frog to the later backing plates/cylinders from a 76 Midget, I had to use the later rods since the ends that attach to the wheel cylinders are rotated 90 degrees when compared to the original ones. And IIRC the length is slightly different but I'm not sure. Done a number of years ago and the old memory is just that, old.
Martin

Martin, that reminds me about recently putting wire wheels on a 1973 Midget. I didn't have the shorter push rods but found a pair off a Frogeye with the ends at 90 degrees to normal. They were, though, the correct length for Midget wire wheels (i.e. shorter than steel wheel rods) so I put them in a vice and with a screwdriver rotated the ends through 90 degrees till they fitted onto the Midget brake levers.

Rob
MG Moneypit

Rob,

I thought of doing that but I didn't know what the twisting would do to the strength of the rods. Being as old as they are I didn't want to risk weakening them by twisting them. Not being a metallurgist, I had know idea if it would fatigue them and if so, how much. I know that it is "just" the parking brake but I have had occasion to use it as my only brake (different car) and didn't want to risk them breaking when I needed them the most. Call it paranoia but I don't want to take the chance.
Martin

Guy

The email address ATwellerHYPHENlakesDOTcoDOTuk does not work. Is this correct. Or contact me on frogeye61AThotmailDOTcom
Bob Beaumont

Not sure why that wouldn't work Bob. Must have got "lost in translation" !!
GuyW

Another !
Rear brake shoes are listed as different for the early sliding cylinder brakes and the later style twin acting cylinders.
But given that the drums are the same, are the shoes interchangeable? The later style are listed at around half the price!
GuyW

Hi Guy

No they are not. They have different adjusters and handbrake mechanisms. They are the same as morris minor. The later spridget ones fit a range of cars including mini/1100 etc.
Bob Beaumont

Hi Guy

Quite a few on flea bay cheapest is 13.99 for a set inc post,
Bob Beaumont

Thanks Bob, I will look at ebay then.

I know the handbrake and adjusters differ, but wasn't sure if that would mean the shoes wouldn't still fit.

Having the additional slots for the later handbrake lever system souldn't make a difference - they just wouldn't be used. I was less confident about the effect of the later style adjusters and the pistons as these may mean a different o/all length to the shoe contact heels. One would need both side by side to compare.
GuyW

To answer my own question - no, the brake shoes are not interchangeable.
The earlier MK1 Sprite ones have a notch in one end of the shoe that locates over the pin of the snail-cam adjuster
GuyW

I have two handbrake levers for my Frog. One long, which I think is the correct one, and one shorter one, like the later cars. I guess I could use either.
The chrome has more or less all gone from the long one. Other than this, are there any other practical reasons to choose one over the other?
GuyW

It has been awhile but I think the backing plate end of the rods from early to later have different size holes. I will have to take a closer look.
J Bubela

Hi Guy, You can use either handbrake but the long one is the correct one. The chrome on mine has gone but it now has a nice nickel coloured patina.
Bob Beaumont

The chrome on the proper one had gone, but it hadn't left a nice patina. :-(
So now it is gloss black with a black rubber tube stretched onto the handgrip part. Looks OK - ish, but I may one day strip it and get it rechromed.
GuyW

This thread was discussed between 07/10/2017 and 05/11/2017

MG Midget and Sprite Technical index

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