Still got brake problems!

[Hans Efde]

Since these are dual circuit brakes, surely if a rear nipple, say, is open, the secondary circuit should still be doing its stuff, if not there isn't much point in having two circuits.

There is quite a distance between the primary piston and secondary. Only when the primary has covered the distance it will start pushing the secondary piston. Because the throw of an unmodified brake pedal in a Midas is about 30% less than in a Metro (thanks to the shallow floor) it needs to be 100% efficient. I did not modify my pedal because it catches almost immediately. I had similar problems as you and found out on my brake servo I could push the lower input rod while the outgoing did not move (the brake servo has 2 rods with the bladder in between). I replaced the brake servo with an adaptor plate and a direct linkage going into the primary brake piston. I adjusted it so that the piston just sits below the vent hole. The vent hole is an open connection between the reservoir and the brake lines and is there for brake fluid expansion. The brakes only will work when the piston has passed this vent hole, so that's another small dead stroke. If the piston sits above this vent hole, the brakes will seize when the fluid gets warmed up, a well known culprit. If the primary piston sits too low the brake pedal has a huge dead stroke. To adjust it, I had removed the reservoir and stuck a needle in it so I could feel where the primary piston was and when it would pass the hole.
Note, the secondary piston hangs on a small pin which is pushed in before the reservoir is placed back, so is always at the right position (unless you forgot to install it ), see the cross section in the Haynes.
If you are completely desperate it would be a good troubleshooting step to pump the reservoir empty and remove it. Then put a needle in the vent hole and notice at what pedal position the primary piston pushes against it. Then you have found the dead stroke of the brake pedal. The smaller this dead stroke the better obviously. I have the feeling on your car the dead stroke will be huge.
To clarify things here are a few pics:

master cylinder.jpg (38.65 KiB) Viewed 11834 times

the master cylinder with the primary piston and the upper piston (right one on the pic) being the secondary

vent hole.jpg (56.95 KiB) Viewed 11834 times

The right hole is the vent hole that is an open connection between the reservoir and the brake lines.

brake1.jpg (30.82 KiB) Viewed 11834 times

My adaptor plate that replaces the brake servo

brake2.jpg (30.68 KiB) Viewed 11834 times

The adjustable push pin that goes straight from the brake pedal into the primary piston

[manifold]

I think the answer Hans gave is the one.

Am I glad I dont have all this anymore. 5 mins max to do both front and back. (with front wheels off)....and I can do rear or front independently...........

[Geoff Butcher]

The annoying thing is that I've been driving this car for 10 years and well over 100k miles with no particular brake problems, other than a new servo a couple of years ago. I've been looking at diagrams, and noticed that none of my increasing collection of m/c's has that stop pin to retain the secondary piston, or the drilling for it. I would be interested to find exactly where the pistons are in the cylinder but after all the bleeding I've done I can't face emptying it out again at the moment.

[Geoff Butcher]

David, what size cylinders do you use? With a pressure limiter? Presumably you have them plumbed one cyl to the front and one to the rear?

[manifold]

Geoff, one thing you need to take into account of is subtle changes in GRP with age. As it ages it does becomes more flexible...hence why older yachts are less competitive as they become less stiff. The hull sags when powered up on the wind causing the mast to dump the power....result is losing forward motion.

You might be experiencing this sag in the bulkhead as you press the pedal....causing the pedal to ground...when it didnt used to...and meaning you are not getting a decent bleed stroke when pumping....comes back to maybe using an easy bleed to solve this issue..

One method I used to do was pump the master cylinder up a few times with nipple closed to get a pedal.......then get an assistant to slowly undo the nipple...then they quickly nip up again as soon as the fluid comes out before it sucks back.

Simple tip to avoid blowing up another reservoir. Loose some tyre pressure first! You can always pump it back after.

I hope that helps.

[Hans Efde]

The annoying thing is that I've been driving this car for 10 years and well over 100k miles with no particular brake problems,

That's indeed annoying. But master cylinders are quite simple. I took mine apart and just cleaned it. I had bought a new seal set, but it was the wrong type. So I just installed the pistons with the old seals since all looked well. No corrsosion, debris or damage inside and seals undamaged.
Is there any chance of excessive play in the brake pedal or clevis pin after such a milage? I can't imagine it would cause such problems, but on my relative low milage car I noted more than 50% dead stroke of the brake pedal travel during my brake problems.

[manifold]

I use a 7/8" cylinder at back and 5/8" cylinder for front. Not brake bias valve.... just a balance bar.

Standard metro wheel cylinders on the rear and KAD single line, single nipple bleed calipers on the front.

[Rich]

I've got a couple of master cylinders kicking about if you find you are missing parts.

Rich

[Geoff Butcher]

Following my comment that my m/c had no stop pin, I had a look at another cylinder which I bought for the reservoir, and it looks like this:


Stop pin present plus two little washers described as baffle plates.

Whereas my other three cylinders have no stop pin:



I wonder why they decided it was no longer necessary?

Got a smidgin more air out today with David's pump and release method

[Hans Efde]

I didn't know they had m/c without the stop pin, it's not mentioned in the Haynes, possibly from another type Austin?
Maybe a stupid question,but have you adjusted the drum brakes shoes prior to starting to bleed? If you haven't they can move too much, so the rear brake cylinders "use" too much brake fluid.
I thought about your remark concerning primary and secondary system, that if one system fails it should still be possible to brake with the other system. It does, but then the primary piston has to travel upwards until it touches the secondary piston which then can be pushed. I suppose that's the clunk you heard on your Ginetta. If the primary piston is sitting too low in the m/c you a) have a long pedal travel before braking occurs and b) it may never get to the secondary piston.