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MG Midget and Sprite Technical - Master cylinder seal sizes

I have sleeved down the Mark 1 master cylinder to 3/4" and have run it like this with front discs for several years. The time came to renew the seals so I ordered the Mark 3 ones as before. Just about to assemble when I found they are smaller than the old ones. They are a loose fit rather than tight as they should be. I thought there were only 2 sizes, 7/8" and 3/4". After nearly 50 years with these cars am I wrong?

While I'm at it, can anyone recommend paint that resists brake fluid? E-Tech caliper paint is very expensive and says it resists oil, chemicals, and brake dust, but doesn't mention brake fluid, which I'd have thought was the most important thing. Don't know what they mean by chemicals as everything is chemicals.

Les
L B Rose

Can't help with the bore sizing Les, sorry but synthetic brake fluid doesn't affect any car paint and is considerably less hygroscopic than conventional brake fluid. The drawback is you should really change all the seals in your current system before filling with synthetic fluid. It's more expensive but doesn't require changing as frequently.

Someone else may have found a heat proof paint which is unaffected by brake fluid thereby rendering the above information useless!
Jeremy MkIII

Do you mean silicone fluid? I heard it doesn't give as hard a pedal.

Les
L B Rose

I'll be using silicon fluid in my newly fitted system (when I get the clutch slave sorted!) so I'll soon see if there is any difference in feel. I can't see how though, surely a liquid is a liquid and you can't compress it?

I'm using it because DOT4 etc would instantly remove my new cellulose paint job!
John Payne

I've been using silicon DOT 5 in my MGB for over ten years. I had put in a completely new system, cylinders, calipers, flexibles and rigid pipes, so there was no compatibility problem. The pedal is certainly not spongy, but is nice and firm and feels no different to any car I have driven. I haven't changed the fluid in all that time until this month when I had to dismantle the rear brakes (Sierra discs) for some other work. The fluid was as clean as the day it had been put in.

One criticism levelled at DOT 5 (by those who haven't used it, usually) is that it is hard to bleed the system, but I haven't found that. I bled the brakes this morning following completion of the work, with Mrs H pumping the pedal while I operated the bleed valves. Dead easy and all done in half an hour.

It doesn't strip paint, but you still don't want silicon on your paint. It's murder to get it off completely should you need to touch up the finish. However, silicon brake fluid is brilliant at making black vinyl look like new.
Mike Howlett

Yes silicone. I've had it in for, ahem 10 years now (should really change it I know) and have had no problems with the hardness of the brake pedal. I fitted a servo when restoring the whole system so don't have a direct comparison.
Like John I liked the idea of it not harming the new paint job when eventually I'd spill it and the better resistance to absorbing water.
Jeremy MkIII

Thanks chaps. I just have this niggling recollection that there were some disadvantages with silicone fluid, if you say not then maybe it's the way to go.

WRT the seals, I'm told the smaller ones are for later cars but as a Frogeye owner I know little about these. I had no idea the cylinders were smaller, I thought they were all 3/4".

Les
L B Rose

I am only aware of the two bore sizes for the A series powered cars, 3/4 and 7/8. Not sure of the size of the 1500 cars.

As regards paint I cleaned my master cylinder back to the original casting colour and sprayed it with petrol resisting clear lacquer. Been fine for the last 5 years
Bob Beaumont

Les,

Maybe try and contact the seal maker to verify the correct dimensions and compare with what you have. It would not be the first time the wrong part has been supplied in a box, I've heard of it happening a number of times with pistons where the piston didn't match what the box said. In one case one piston in a set of 4 was the wrong size, I don't know how that was resolved as they were sold as sets.
David Billington

I was told recently by someone who has done it, that you don't need to change the seals when swapping from dot4 to silicone fluid. Merely drain the dot4 out, flush the system with Methylated Spirit, allow it to evaporate,then fill with silicone. But I cannot vouch for the effectiveness of this procedure, as I've never done it.
I suppose that if it was no good, you could always drink the rest of the Meths.😂😂.

Bernie.
b higginson

Instead of painting you could apply Kurust.It leaves the casting with a kind of purple/black anodized appearance, and seems to be a fairly hassle free instant solution.
f pollock

This thread was discussed between 30/03/2019 and 02/04/2019

MG Midget and Sprite Technical index

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