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MG Midget and Sprite Technical - Air in brakes

Hi folks,

I have an annoying issue with the brakes. Single circuit early 1500.

I can bleed the brakes and get a firm pedal but it only lasts a week or two of use before the pedal starts getting soft. I can still pump and get a firm pedal.

Twice this has happened and I have re-bled the rear cylinders and a few air bubbles have come out. The firm pedal returns, but only temporarily.

I don't appear to have any leaks (certainly not from the cylinders or calipers).

Any ideas why this keeps occurring?

Cheers,
Malcolm
Malcolm Le Chevalier

Have you checked the bleed nipples are still tight after using the car, you might need to check a couple of times after use - and perhaps have some grease to seal the threads when you open them to bleed them to stop air being drawn in.

I hope my post draws other idea for you too.
Nigel Atkins

Thanks Nigel. I will be having another go tonight and will check the bleed nipples in the process.

Malc.
Malcolm Le Chevalier

I got leaks from new bleed nipples I put on and found I needed to nip them up a couple of times after runs but then they settled.

The new rear cylinders on mine came with 7mm spanner size bleed nipples so I bought a 7mm to be sure.

I'd also suggest you check all connections for nipping up, don't be brutal and break them but see if they nip up a little, the roads are now so rough that I think they can shake things loose.
Nigel Atkins

Tightened up all the unions the other week. Bled them again.

A week on... going soft again! Grrrr!

Seem to have narrowed down the issue to the rear near side at least.

Malc.
Malcolm Le Chevalier

If its that corner of the car there aren't many things to go wrong, leaky slave cylinder, bleed nipple, or the T piece mounted on the diff maybe ?
C MADGE

Could be worn wheel cylinder cups sucking air but not worn enough to leak under pressure
I've had it happen with disc calipers before
willy
William Revit

By coincidence when I had to changed the rear cylinders last year I still had a leak which appeared to be at the joint from the pipe to the new cylinder on the near side.

I replaced the pipe (c/w unions) from t-piece to cylinder and this stopped the leak.

I released the t-piece end of the pipe first and plugged the t-piece with a new pre-cleaned setscrew (IIRC 7/16" UNF) whilst I shaped and fitted the new pipe. This worked without the need for stage bleeding and held pressure.

I was prepared for stage bleeding and even bought another new cylinder in case the threads on the existing new cylinder were the problem.

I used one man and jar method again without issue.

Same as Malc I'd previously tightened all the unions and checked them again and the new nipples which needed nipping up a couple of times.

I don't know why changing the pipe was necessary but probably like Malc by the time I got to it I didn't care and just wanted no more leaks. It was the same with the fuel tank, a leak from the union on the tank after trying all else. I did wonder if the rough state of the roads might contribute to vibration wear as I've also had to tighten the odd fixing on the car so I softened damper settings, too early to tell if it has helped.
Nigel Atkins

I think when new copper or kunifer pipes are used care should be taken not to overtighten the unions as it distorts the flared ends on new pipes. Nip them up lifhtly and they should seal. If there is a bit of a weep they can then be tightened a bit further. Overtightening to start with crushes and distorts the flared ends which then leak and cannot be fixed by further excessive force.

Try dusting some talcum powder around pipe unions which will darken if it soaks up any slight moisture and make it easier to spot any leaks.
GuyW

It is a new cylinder on that corner after the previous sprung a leak. So something defiantly not right there...

Ho hum... I will have another go. Might need brakes for the weekend!

Malc.
Malcolm Le Chevalier

ETA: mine were new cylinders when I needed to change the pipe or union


Totally agree with not over-tightening, a small leak or sucking-in is usually more convenient than broken or spoiled parts.

With the leak I had the fluid appeared on the inside of the backplate (and over my newly fitted shoes) which would suggest the cylinder usually but it wasn't in this case. Leaks, and air sucking-in especially, can be a devil to fully track down.
Nigel Atkins

Leak with 2 different cylinders would suggest a damaged flare or even a slight crack in the pipe where the union nut covers the pipe. Try making up a new length of pipe with fresh flared ends.
GuyW

The leak was from the cylinder seals rather than the pipe-cylinder joint Guy.

xxxxx
Malcolm Le Chevalier

ha ha ha, excuse the kisses! I was writing an email to my wife at the same time! ha ha ha!
Malcolm Le Chevalier

Oh yes! Well I am glad Aberdeen is a good long safe distance way away!!

Sounds like poor quality wheel cylinders then. Or unless your brakes are adjusted very slack and one of the pistons is moving too far out of the bore.
GuyW

Malc,
I had a leak with what at least appeared to be the cylinder and with pipe/cylinder joint.

It's possible I might have caused the second leak but I'm careful not to over-tighten having learnt from that mistake decades ago.

Or perhaps the first and last leak was caused by the pipe or union and I didn't need to change the cylinder first time, but there was a lot of fluid on the inside of the back plate and over the shoes.

TBH with the first leak it happened at a very inconvenient time, as they usually do, and I didn't fully investigate I just jacked up the rear wheel and as I was able to turn it by hand with the handbrake partially on I just ordered the cylinders and shoes.

After I had my usual Sod's Law episode with things that don't seem to happen to others (new leaks from new nipples and no vent hole in reservoir cap).

If you have the kit and sufficient pipe you could cut the end off the existing pipe and put on a fresh flare, that would only leave the union nut (or the new cylinder or the combination of both).

xx
Nigel Atkins

This thread was discussed between 09/08/2018 and 22/08/2018

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