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MG Midget and Sprite Technical - Tandem M/cyl problems

I'm trying to help a friend with his Frogeye that he has changed from drums to discs. He has got a new 3/4" master cylinder. At first, before I got involved, he had binding brakes. He managed to free them by shortening the push rod, but now we cannot get a decent pedal. In fact, even after bleeding with an Ezeebleed, the pedal refuses to remain firm. It can be pumped up and the pressure holds, but if you release it, it then goes to the floor. It feels like air in the system, but no air is coming out. I had to leave today, but I'm going back tomorrow, so I thought I ask for any ideas and advice on here before I go. I'm starting to think the new m/cyl could be faulty.
Bernie Higginson

Did you change the rear cylinders?
Iirc they differ for drum and disc brake cars.
Could give you a very long pedal.

But the twin MC's are notoriously hard to bleed
Onno K

There was a thread recently about problems with TRW make. The better ones are the AP/Capero ones.

Was the shortening done at the MC end by the adjusting nuts?

Don't shorten it too much, just enough to prevent binding brakes.

If they were binding before the adjustment then the system must be fluid tight so I would suspect the MC.

Are the front brake calipers on the correct sides? I had a mini once that were an absolute pig to bleed until I realised they were swapped left to right so the bleed nipples were upside down.

Rob
Rob aka MG Moneypit

Rob. The M/cyl is a TRW bought from you know where. Calipers are on correct sides. Alan has got long and short push rods.

Onno. The rear cylinders have not been changed, but although it is advisable, I know people who have had no problems with using the originals with discs.

One reason I am suspecting the master, is, if I leave the Eezibleed connected and pressurised, the pedal is good and doesn't collapse, but the front brakes then bind. Release the Eezibleed and the pedal collapses.
Bernie Higginson

Bernie
The two main complaints with TRW tandem cylinders are residual brake pressure or leaking.
Tandem brake cylinders are not difficult to bleed.
Why do so many on this BBS make such a song and dance over the subject? Had tandems on my Frog for 47 yrs of ownership.
If fitting a 3/4 master cylinder to a frogeye with standard rear drums you need Morris Minor 1962+ slaves to counter a soggy pedal.
If needs be I can search out the part number for you.
Did you sort your FWBs.
Alan
Alan Anstead

Alan. Yes, I've never had any previous problems with bleeding the tandem cylinders. I'll pass on the Morris Minor info. In fact, my friend with the Frogeye has a Traveller, so he may have some spares.

Yes, sorted the wheel bearings out. Thanks for asking.
Bernie Higginson

It does sound like a master fault

i wonder if the caliper piston is sticking... that happened to me once...finally had to replace the caliper... the piston is high carbon steel and not chrome plated or stainless and will rust at a drop of rain

prop

Bernie,
The standard dual M/C has a spring&valve deep inside the brake side which retains some pressure in the system, acting against the shoes return springs, to keep shoes in light contact with the drums.
As discs don't have return springs this residual pressure will result in them binding.
I've no experience of the 3/4" M/C but it sounds as if you may have the same problem.
I had the same issues when I converted my frog to discs and was at a loss as to what I'd done wrong. But a search on this forum gave me the answer and removing the valve sorted the problem.
You will need a long pair of needle-nosed pliers to grip the valve.
Dennis
D Stanfield

Thanks Dennis. The 3/4 cylinder also has this valve, but of a different type. The 7/8 one has a residual valve but the 3/4 one has a restrict or valve. But that may be what the problem is.
We have several choices to play with regarding 7/8 and 3/4 cylinders, remove the valve, shorten pushrods etc.
Bernie Higginson

A bit rushed for time sorry, so brief - if you check the archives I've made a fuller comment before:

If the caliper seals are a bit sticky on the pistons, they will retract the pistons quite a bit when the pressure is released. 0.5 mm per piston is enough to use up all the stroke of the master cylinder.
Cure if this is happening: strip, clean and re-lube the seals & pistons, assuming all in good order.

Happened with me on new seals and rubber grease which was a bit to old and tacky as a result. Took hours of frustration to establish the cause.
Paul Walbran

Bernie, when I put a new one on mine I had to adjust the pushrod quite a bit. The other thing I did was put a new return spring on as the pedal was not coming back up right to the top of its travel. This was probably not helped by the fact that while I had the pedal box out I painted it all but its worth checking that the pedal is coming right up.

Trev
Trevor Mason

Thanks to everyone for their advice and comments.
As the TRW master cylinder could be faulty, we took Alan 's advice and bought Morris Minor rear cylinders, but kept the 7/8th master cyl. I left out the valve because it was he wrong one for discs. We played around with push rod adjustment and we now have a good clutch and a reasonable brake pedal. I think the brake side of thing would be improved by fitting a 3/4" master cylinder with the correct restictor valve, and further improved by doing as Onno suggested and go the whole hog and fit the later rear back plates and twin piston cylinders. But first we have to go to France for the Le Mans Classic.
Bernie Higginson

I like the cavalier way you say, "strip, clean and re-lube the seals & pistons", Paul.

I've been trying to blot out the memory of drifting those beastly tin dust seals in square. I certainly used two sets of four, to get one set in undamaged.

Nick and Cherry Scoop

Yes, they are a mission! But you'll be please to know that for that particular exercise the seals don't need removing ... all the cleaning that needs doing can be done with them in place.
Paul Walbran

This thread was discussed between 27/06/2016 and 01/07/2016

MG Midget and Sprite Technical index

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