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MG Midget and Sprite Technical - Rear brake back plates?

Now that the front is on stop it's time to have ago at the rear brakes. I have had the brake cylinders leaking pretty badly, so I've got all the parts but I want to take off the back plates to give them a good clean. Do I need to remove the half shafts to get the back plates off? Would I be better off leaving them on and cleaning them in situ? I also thought that with the back plates off it would make it easier to remove the old wheel cylinders and fit the new?

Thanks
Kevin
K Grice

Kevin,
Yes you would need to remove the half shafts and rear hubs to fully detach the backplates. So I would say it depends on how bad they are and how much cleaning/ repainting you plan on doing.

A good upgrade is to drill and tap your new cylinders and to fasten them in position in the backplate with small locating screws. Makes for more positive and accurate brake shoe adjustment.

Guy
Guy Weller

That's a sizeable job, Kevin. Basically, everything off right back to the bare axle tubes, including the hubs - so you'll need that elusive 1 7/8" socket, and you'll probably decide to fit new bearings, and, and, and.
Nick

That'll teach me to make tea in the middle of writing a reply.

Wot Guy said.
Nick

Still think those cylinders should float.
Jeremy Cogman

Jeremy,
I am sure that you are right and the design is intended to "float". It is not accidental that the cylinders move about and in theory it should make them self aligning.
I just don't find that the design actually works that well in practice. But fastening the cylinders securely to the backplate does seem to help.
Guy Weller

Tell you what really cheers a chap up: - taking so much trouble over getting the new bearings right, and knocking the hubs back on in the correct way, and doing up the hub nuts nice and tight, and bending the new lockwashers over the flats . . . .





. . . that he forgets to re-fit one of the backplates.

Nick

Hi, no hijack intended. I'm tending towards fixed cylinders. Look what happens when floating, nicely milled flat on the cylinder body and scuffed seal.
mark


m Marston

Thanks for the advice guys, all is going well apart from the oil drip comining from the half shaft gasket.
Is this easy to replace, I know you have to drain the oil, but is it a case of pull the half shaft out , replace the gasket & push it all back together?
K Grice

Can you not just jack that side of the axle up to stop the oil flowing out?

Carl
C Bintcliffe

There's an O-ring and a paper gasket.

I always find it difficult to get a good seal there, once the wheel nuts are no longer pulling half-shaft and hub together. The replacement O-rings always seem a bit too perky, and the face of the half shaft has usually been knocked about a bit after years of previous owners prising it away from the hub with a chisel.

Do people use gasket sealant?
Nick

No but I always grease the gasket.
Jeremy Cogman

Agree with much of above.

Half-shaft seal tends to leak once wheel nuts removed; esp if the drum is heaved off with gusto.

Temp cure is to tip the axle the other way (never overfill axle...!)

Re-furbish the back-plate in situ.

Half shaft normally withdraws easily without tearing gasket; I rarely replace gasket and o-ring unless there's an obvious problem.

I don't use sealant.

I believe that the s/cyl is meant to float, so that the s/cyl and shoes find their optimum position in use. The servo effect is very sensitive to the correct location/alignment of the s/cyl and shoes wrt the drum and adjuster pivot points. (I once spent several hours
studying a comprehensive engineering analyis of drum brake system - it has more complex geometry than disk system.)

HTH A
Anthony

I disagree that the wheel cylinders should "float" The 1971 came with a large nut behind the cylinder that secured it to the backing plate with no movement. The snap ring type came later as the only available replacement. Drilling the hole and screwing the cylinder down firmly makes a noticeable difference in tightening up the shoes and eliminating "slop"
Glenn

Thanks for your comments, I have decided to leave the back plates on.
On another note linked to this, heres what else i'm going to do to the rear end: New polybushes & spring pads, lowering blocks & U bolts, new shackles, new brake hose, new handbrake clevis pins & washers. So I was thinking maybe I should remove the whole rear axle, as by the time I remove all the above, the only thing holding it on would be the propshaft (4 bolts), and the check straps. This in theory would allow me to remove my stubborn axle filler cap (rusted) more easily, be able to secure my brake cylinders more easily and allow me to paint it in comfort, rather than lying under it. I can make removal easier by grinding off the U bolts & shackles. Is it a good idea or not?

Kevin
K Grice

Kevin yes take the whole shebang out. Drain the diff first. The U bolt nuts should/will come undone easily (or will shear in seconds), probably quicker than grinding or cutting them off.
David Smith(davidDOTsmithATstonesDOTcom

Hi every one, New to this site and a bit of an amateur but in mark marston's pic above, isn't LH shoe misplaced ie. a little low in relation to the cylinder, and is this why they have a 'milled' flat on cylinder? or is it just the picture.. or is it just my eyesight? On second glance shoe doesn't appear to fit in the cylinder locator at all well. Any comments, please let me know soon as I'm doing my own r. brakes next week. soory mark. Cheerio for now, MJB.
MJB

I do not know whether the original engineer intended the cylinder to float or not, but I have had results similar to Mark's picture before I started threading the roll-pin hole and screwing the cylinders down. I really hate pulling the drums after a couple thousand miles to find slots cut in both dust covers.
David "only slightly screwy" Lieb
David Lieb

Hi,
Thanks for the comments on the picture, I'll take a look at the shoe fitting. Can't do it 'till the weekend though, got a bad back at present (there goes my international polo career)
I'm on my spare dust covers at moment, last lot got holes like David.

On original point, I've only installed the halfshaft gasket 'dry', but I notice it does weep a bit.

M
m Marston

Regarding Mark's photo above, I suspect that the shoe has simply been pushed downward after the drum was removed, and wasn't actually working in that position. With the shoe moved down that far, the drum probably wouldn't fit anyway. As you know, the shoes are free to slide up and down easily when the drum's off.

However, that has nothing to do with the wear that's apparent on the cylinder and dust seal. I know a lot's been said pro and con about cylinder float, but be real, they don't float THAT far! There's really very little movement with the roll-pin arrangement after all.

Just my tuppence,

-:G:-
Gryf Ketcherside

the movement is not in the plane of the backplate but canting over upwards, which is what puts the rubber cover in contact with the inner edge of the drum. I believe it's exacerbated by using those horrid C clips instead of the NLA Belleville washer and proper circlip. In the first incarnation of racer we used to suffer deep scoring of the rubbers and eventually the movement was so extreme a piston popped out and I wrote the car off. Needless to say they are bolted in now....
David Smith(davidDOTsmithATstonesDOTcom

Rear axle is all back together now with cylinders bolted on I removed the roll pin, tapped the hole and hey presto! Thanks for all your help.

Kevin
K Grice

This thread was discussed between 20/02/2008 and 03/03/2008

MG Midget and Sprite Technical index

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