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MG Midget and Sprite Technical - Rear Brakes advice

I do hope someone can help me with some advice!

Sadly, my 1973 1275 failed its MOT today when the nearside (left) rear wheel locked under the rear foot brake test. I had adjusted the brakes last week and gave it a good run and a few hard braking tests afterwards and all was well, as was my 400yd run to the MOT station toady. Tester said the slave cylinder had failed.

I got back home very slowly with the wheel almost locked solid - couldn't push it. I put it up on the jack, thinking the handbrake linkage might have got caught up when reversing with a mighty jolt onto the rollers, but that was free, so I loosened the adjuster a fraction of a turn in readiness to remove the hub and the wheel freed itself immediately.

Interestingly, this happened last year when the same wheel locked up when leaving the MOT station (having passed OK). It was freed in the same manner back home and has given no trouble at all for a year.

I have checked slave cylinder, linings, springs and adjuster and everything else I can, and all appears to be OK. Incidentally, in case anyone asks, the linings and springs are all installed the right way round.

To me it points to locked brake shoes - but why? Anyone have any bright ideas, otherwise I'll simply take it back for a re-test as it is, with the fear that the problem may recur.

Here's a couple of views of the offending hub.

Any advice would be welcome, to save me pulling the remains of my hair out.

Peter



Peter B

Could it be one of the adjuster wedges getting snagged? Looks as though they aren't sitting very square in their slot.
r thomas


Hello Peter.

When you adjusted your brakes did you lock the wheel & then adjust 3 notches back?

Are you drums clean & unwarped?

If the cylinder does need replacing you have to prise
off a W ciclip & getting the new one back on requires the presense of Houdini. There is an aternate option involving drilling/tapping & then bolting. Houdini's mum could then fit it. See the archive for this.

Gavin Rowles

A few of thoughts.
1). The adjusters are sq sectioned. Hence there is a distinct feel as you turn the adjuster between the 'flats'. Had you settled the adjuster on a flat? Is the adjuster free to rotate on it's own? Seems very unlikely if the bottom spring is good and strong. Are the adjuster wedges free to move in and out, not get stuck out? Again, not likely as they are held in by the spring on the shoes.

2). Is the wheel cylinder in good shape? Can you 'easily' move the pistons in and out of the bores? No rust? Nothing preventing the piston returning into the bore properly? Again seems unlikely with a good spring.

More likely is this.
3). The rear brakes are SLIDING twin leading shoe. As you say they are mounted correctly, and this makes them mostly self aligning inside the drum. Did you also know that the leading edge of the shoes 'assists' braking, by grabbing hold of the drum? But because the shoes slide on the adjusters, they don't tend to grab too much, and don't tend to SPRAG(wedge the shoe against the drum) as it's termed.

If however something is stopping the shoes from sliding on the adjusters, a rough or bent edge, then the leading edge of the shoe(s) will grab against the drum and wedge there.

Also, if the shoe linings are out of shape, so that too much of the leading edge is in contact "before" the rest of the lining, this can cause the lining to be wedged agains the drum too.

Check the faces of the shoes against the pistons, and in particular the adjusters, and make sure they are free to slide back and forth. Also make a very small chamfer on the leading edges. Not too much otherwise you remove all of advantage of leading edge.
Lawrence Slater

Thanks for your useful comments. Yes, I wondered about warped or distorted drums, but difficult to check. I also wondered if there was excessive drum wear, but again, very difficult to check the inside measurement sufficiently accurately to know if it's within tolerance.

Lawrence, yes was I was aware that the leading edge starts the braking process, and the symptoms I have do seem to be what you describe as SPRAG (never heard that expression before!). Quite a few more points to check there. I also found the old Mintex shoe installation instructions which recommends a small amount of high melting point grease be applied to the sliding/pivoting points. Will try that too.

I will also try applying a small chamfer to both leading and trailing edges, because i suspect that the linings wedged under braking whilst reversing when the trailing edge becomes the leading edge.

Oh, and the adjusters were backed right off when I took the photos which explains why they may not be sitting very square in the housing.

As I said, more to investigate, but I really do appreciate your thoughts.
Peter B

I'd agree with with Gavin.

I have an MGB based kit and a Sprite all my numerous MOT rear brake problems have been down to me adjusting the rear brakes too close, getting the rear brakes balanced has seemed to be the most successful approach. Seems to be an issue with the way the brake cable pulls the two sides differently.


Malc Gilliver

The only thoughts that I can add to the comments: looking at the photo of the shoe pair in situ, there doesn't appear to be any grease on the shoes' ends (where they fit in the piston grooves or at the adjustment screw end). A spot of grease helps the 'sliding action'.

Also (shoot me down if I'm wrong, it's some time since I worked on my midget rear brakes). In the Midget design, does the edge of the shoes rest on the backplate at all? (I really can't remember). I know they do in my Astra (sorry about the language!) - but I can't remember if they do in the Midget. If they do, again, a tad of grease helps the sliding action of the shoe edge against the backplate.
Andy Hock

Has anyone already mentioned checking the inside of the drums to see if there is a significant lip, either from wear or build up of crud. This might catch and drag the shoes on to make the brakes bind.

And what about the brake equaliser - the Christmas tree thing on the rear axle - is it free to pivot?
Guy W

Thanks for your further advice.

It is possible that I had adjusted the rear brakes too close, but will make sure I don't when I reassemble the brakes. Rear brake balance and "Christmas Tree" operation is fine.

I did check the wear on the brake drums after removal, having located an appropriate pair of callipers, they are well within tolerance and appear to be true.

I take on board the greasing recommendation, which I already knew about. When I did the rear brake overhaul a few years' ago, I reassembled everything dry, but will apply small amounts of grease this time. I propose using a smear of Copper Ease unless anyone can suggest a more appropriate product.

When I removed the shoes, I saw that there were signs of light contact with the backplate, so I'll consider giving that a very light smear too, but I'm concerned that it could get onto the linings.

When I removed the shoes, I noticed that one of the pistons remained about 5mm further out than is opposite number. I unsuccessfully tried to compress it back, so wired the pistons loosely together and with the help of my lovely assistant, applied the brakes. There was no movement at all, so it appears that the slave cylinder has locked up. The cylinders I purchased at the time (from the well known parts supplier that is not gathered by a rolling stone) were not originals but cheaper alternatives with plastic pistons. One was faulty straight out of the box and had to be replaced.

I've now ordered a pair of genuine AP slaves from MiniSpares and will fit them as soon as they arrive . Hopefully that will sort the problem out.

I'll let you know.
Peter B

Copper Ease is good. Just an 'angel's kiss' of grease on the backplate. Don't overdo it.

Re. rear wheel cylinders. Plastic pistons?? WTF?? Surely you must be mistaken? Obviously the Chinese don't understand the forces and temperatures involved with heavy braking.
Andy Hock

Peter,
I guess you mean the supplier was T*ss

I've conversed with a chap in their technical department and one of their higher level customer managers and it appeared to me that they'd become company ers
Nigel Atkins

When you fit the good cylinders and adjust the brakes do you help the setting of the shoes?
Each time you lock the brakes with the adjuster give the drums a bash with a copper or hide hammer on all 4 quadrants.
Then check if you can adjust it tighter.

Works for me every time
Onno K

Andy

Yes. I was mistaken. They looked like plastic in situ but having removed the offending cylinder, I see it is steel. Interestingly the pistons in these non-original cylinders can't be rotated and I wonder if this has contributed to the problem.
Peter B

Peter. The pistons in the original cylinders can be rotated too. The stay in the 'vertical' position though, due to the slot that takes the brake shoe.
Lawrence Slater

Peter B, thanks for confirming that the brake wheel cylinder pistons are not plastic (although some may argue that far east 'monkey metal' is not much stronger than plastic).

I do not understand why your brake wheel cylinder pistons do not rotate. AFAIK, there is no reason that they shouldn't. I would go so far as to say that they definitely SHOULD be able to rotate in the cylinder bore such that the pistons can find their own correct, unstressed orientation when the brake shoes are fitted. If the pistons can't rotate, they cannot find their unstressed position when the brake shoes are fitted to the backplate. It sounds like there is something peculiar with the brake cylinders you bought.
Andy Hock

Oops. I read can't, as "can" rotate (spec savers needed i think). So I completely agree with Andy.

Can you pop out the pistons and post a pic?
Lawrence Slater

Lawrence

Here is a picture of the offending cylinder and as you can see, the right hand piston is not fully retracted. This is what led me to conclude that I had a seized cylinder. I cannot turn either piston using the spanner shown in the second picture.

the second picture is of a spare cylinder purchased at the same time as the other two. The seals on this one leaked as soon as it was installed and replaced FOC by M**s. I was unable to turn either piston using the spanner but eventually managed to pull the piston free using a vice and lots of twist and pull. When re-assembled it now will rotate. Although it was still well lubricated with brake fluid, you can see marks where the piston has probably stuck in the bore. I think the manufacturing tolerances of these particular cylinders was much too tight.

I'll see how I get on with the new "original" ap ones when I get them fitted and bled. I'm not hurrying as I'm waiting for some new parts to fit while I've got the brakes stripped.


Peter B

Well since the pistons are at least partially siezed anyway, and thus scrap, you can belt one of them with a hammer to drive the other out, and take a peek inside.
Lawrence Slater

>>Lawrence suggests>> you can belt one (piston) of them with a hammer to drive the other out, and take a peek inside.

>>AH>>Agreed - and get a micrometer onto the diameter of the piston and, ideally, the internal bore of the cylinder. And compare readings with those of a new cylinder (1500 Midget bore is 11/16" I think?).

Irrespective of the above - I think you've found the cause of your locking rear wheel!
Andy Hock

I buy and will buy all brake cylinders at:
Company Name: C&C Parts Bv
Product/Services: Brake parts, master cylinders, wheel cylinders, calipers, etc.
Contactor: Mr. Wiel Claessen, Director/CEO/General Manager
Address: De Voort 5 A, Baarlo, Limburg, Netherlands,Netherlands 5991 DC
Phone: 31-77-4772454
Fax: 31-77-4771676

www.ccparts.nl

They are not cheap but the specifications are much better than original: better tolerances, better materials, it is impressing; it last much longer than the m*s scrap. So it is cheap.

Flip
Flip Brühl

PeterB - what make are the 'repro' rear wheel cylinders? Did they come in a box? Any logo or manufacturer's name? Or part number? And a question - from your photo, there seems to be a piece of cardboard on the bleed nipple face of the cylinder? Was it supplied like that? Cheers . . . .
Andy Hock

Andy, yes that's a paper gasket, designed to prevent the slave cyl rusting to the backplate. Optimum handbrake performance relies on a little articulation of the cyl in use.
David Smith

David, thank you for your reply. Yes, I now see it listed in the parts book "37H4642".

And, of course, your (probably correct) assertion of "optimum handbrake performance relies on a little articulation of the cylinder in use" will, again, start the old chestnut: "to tap and bolt the rear wheel cylinder onto the backplate, or use the circlip . . . ?"

(Come to think of it, the designed intention of using the paper gasket rather suggests that the wheel cylinder should not move against the backplate!)

Cheers. . . .
Andy Hock

Not unusual - and not an issue with the cylinder - when did you last change the brake fluid?

Brake fluid is hygroscopic - ie it attracts moisture - and what you seem to have there is rust in the bores due to moisture in the fluid.

Just fitting new cylinders will be a waste of time unless you also change the fluid.

Should be done every 2 years ideally.
Chris at Octarine Services

Thanks for all your comments.

The brake fluid was changed last year, but it is possible the damage, if hygroscopic brake fluid was the cause, had been done before then.

The repro ones did come boxed with AP Lockhead on the box. Part number from the invoice is GWC1102 described as, "W/CYL REAR 63-74 S&M. The invoice shows that I last changed them in 2003!! How time flies!

Anyway, both rear cylinders have now been replaced, the system overhauled and bled. The rear brakes are now functioning properly and the car successfully passed its MOT re-test yesterday.

Thanks to everyone for their help and advice.
Peter B

This thread was discussed between 31/07/2013 and 09/08/2013

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