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Hi, have a totally std 62 850 Austin, needs a new master brake cylinder, all wheel cylinders seems fine, but needs pumping 3-4 times every time i need to stop, so asume its the master. Trying to look a bit ahead, probably fitting discs later, so will a S master fit my system as is now ? its bog standard, single leading shoes and all.

Before you " write off " your master cylinder , have you tried adjusting all your brakes??
Also , if your brake hoses are original , they may be soft and expanding .
What I normally do is clamp all four hoses close to the subframe , pump the pedal and see if there is any difference, is the pedal any better?
Move the clamps closer to the wheels . If there is no difference , adjust all four wheels.
If you find a difference when you moved the clamps , you probably need new hoses .

Craig

Hi , yes, tried that already, all four corners adjusted, and all looks fine. Haven't tried with the hoses, but didn't suspect those, as pedal isn't soft, just needs pumping 3-4 times before anything happens, then it brakes fine, but looses pressure instant again. Kinda like when they needs adjusting, but they don't, they all are fine. Must try tjecking the hoses first. Anyone know if the S master fits straight on, if it needs replacing ?

Holm

that really sounds like the seal in the master is worn and allowing air past itself when the pedal is pumped.

the s master is a straight swop

rich

Personal i'd still drop a set of Braided lines on

Sounds more like a weepy wheel cylinder if you need to pump the pedal and then it works . Doubt you would get any real pedal if the master cylinder seals were shot. Cheers. Mark

Ok , is the pedal low on the first pump and gets higher with each consecutive pump ?
If so , you have " play " in the system .
An easy way of testing a brake or clutch master cylinder is , remove the brake pipe and install a bleed nipple from the front brakes. Drums or discs it shouldn't matter as they should all be 3/8 NF
With the bleed nipple installed , you may have to bleed a " little" air out then close the bleed screw.
Push the brake pedal and see what happens .virtually no movement of the pedal indicates master is ok.
If the master cylinder slowly sinks to the floor , then the master cylinder seals are worn and " bypassing" allowing fluid to return to the reservoir. Replace seals or complete master cylinder.
And yes the "s" master will bolt right up.

Craig

Both brakes and clutch now not working, grrr.. Been busy, but just tried the trick with the bleed nipple on the masters, they both seem okay, so guess a new clutch slave and hose, and new brake hoses hopefully cures all problems. Should be driving instead of holding still in the barn, in this nice weather Thanx for advice

Cant remeber if the early single lead setup have them ?

But the small Xmas trees that hold the piston aginst the shoe, is always missing on the Mini,s that yurn up in my workshop with poor pedal.

Single leading shoe front brakes do not have the 'xmas tree' springs, the set up is exactly like the rear brakes, but I know what you mean about missing ones being the most common reason for bad pedals on twin leading!

update, been busy, so havent got around to it before now. Brake hoses renewed, and rear brake cylinders too, result : same S...
But as we bleeded the brakes, i noticed that when brake pedal is realesed, all the brake fluid returns to the master cylinder Could it be, just an idea, that the owner before me, it has been like that all the time i had it, could it be, that he renewed the master, and perhaps had a used clutch master that he toke for a brake, and put that on ? the clutch should, i guess, release all the fluid back, when depressed. Getting a bit tired of the old girl here...tempted to that new S master, and S discs...

The reason the fluid returns into the clutch master cylinder is because the clutch arm spring pushes the piston in the slave cylinder back, not due to design of the master cylinder. If your brake master cylinder is pulling the fluid back when bleeding the brakes, as most of them do in my experience, you will never get all the air out without a pressure bleeder, it's probably the most useful tool you can buy if you plan a long & happy relationship with Minis! Drum brakes are fine while they are working well, I use them, but once they start to play up in this way, personally, I'd go for the 'S' master cylinder & discs, but that is just an opinion

There's supposed to be (what they call) a residual pressure valve in the Brake MC to stop it doing just that. The Clutch ones (for obvious reasons) don't have them.

Put some clear tubing off the bleed nipple you're working from, in to a jar and see what the fluid is doing there.

One other thing to check is that the MC Push Rod is returning all the way, if it doesn't, even by half a millimetre (with most of them) it will tend to do just what you are finding. One way to check is with the pedal returned, see if the clevis pin is free. Don't rely on the fact that the pedal maybe returned. If you find the pedal has returned and the MC isn't, then the MC will likely need a shim under it, they were used on some models.

The Residual Pressure Valve is a bit of a misnomer with that name, certainly in the case of the Brake MCs fitted to Minis. Rather than actually keeping any Residual Pressure, they are more of a resitrictor to slow the fluid flow back in to the MC. I agree with the good Lord ^ that pressure bleeding is best, however, I found best results when using the pressure bleeder to 'prime' the system with that tool, but then go back to the old fashioned way (Down, (hold, bleed off, lock up), Up, (pause) Down etc) the best way, especially with Tandem Systems.

Don't try to use a Pressure Bleeder that works in reverse. Some that I have seen on the net seem to push fluid back from the wheel cylinder in to the MC! You may damage the residual pressure valve trying as well as making a big mess.

There will always be some reversal of the fluid movement in the bleed line, clear tubing will let you see how much. The rhythm of pumping the pedal will make a difference. I usually manipulate the pedal by hand rather than foot. I use a slow push down and a relatively quick release. What you're looking for is sort of a two-steps-forward one-step-back progression in the line when bleeding. Ideally, with the right rhythm, you'll get more than two-steps forward.

When I bleed brakes I usually lie in the drivers seat with the bleeder open and look under the car at the bleeder line and manipulate the pedal with my hand. Over the years it's proven to be the quickest (although not fast by any means) and most reliable method. Your mileage may vary...

Kelley

I agree with what Spider says, but I just wish the residual pressure valve would actually work, all this stuff comes from personal experience & I have never found any difference between bleeding clutch & brake hydraulics as regards the fluid being drawn back in when the pedal is released. I only ever use pressure bleeding now, unless fitting a new master cylinder, as I am told that pumping the pedal pushes the piston seals over a part of the bore which is not used & may be pitted, causing damage to the seals. I would certainly never use a pressure bleeder that works in reverse.
mascher, that is exactly the technique I used to use, but for reasons explained above, I don't do that any more.
There is an exception to all this, my Mini has the big old dual circuit master cylinder (GMC167, I think) that was quickly discontinued, it bleeds perfectly with no suck-back & when everything is adjusted correctly, pedal travel is about 1 inch. For some reason, it was not well thought of, but I wish I had a shed full of them to replace the 'squidgy' thing that followed it.

The residual pressure valve is not there to help bleeding.
It is there to hold a small system pressure (approx 4psi) to prevent piston knock-back in normal use.

I agree with Spider regarding the pedal/push-rod return.
-The return spring on the pedal should hold the pushrod back under slight pressure, so when you're lying on your back in the footwell trying to rotate the pushrod to see if it's free, you'll need to press the pedal a fraction