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MG MGB Technical - Early B brakes and their foibles.

I have been referred to your site, from "GrassRoots Motorsports" forum.

I am working on a friend's early B. Replaced the master cylinder, shoes, lines, flex lines are now stainless braid etc.

Bled the lines, starting longest to shortest, then back again.

Get a nice solid pedal, then if you let it sit a half hour to overnight the pedal will go to the floor the first time, then you will be back to a solid pedal,

No leaks, no puddles.

Despite having thirty years experience working on cars of all types, I'm stumped.

What am I missing?

Thanks everyone.

Steve from the USA
S. Spangler

This is usually master cylinder. If it's new it may need re-honing to get the seal to fit properly.
Stan Best

Check the adjustment of the rear shoes. Tighten the adjuster in until the drums locks up then back off 1/2 turn. This will eliminate the rear brakes from being the problem. Also, you could have a sticking caliper piston on the front brakes. Ray '67 early B

Hi I had the same problem , try bleeding brakes with the engine running ..

Not straight to the floor on mine, nor do I have to wait as long as overnight, but after 'normal' bleeding I get a long pedal which pumps up, then if I leave it a some minutes and push it again it is long again. On mine this is caused by there still being some air in the system, which I can never get out by pressure bleeding even with a gunson's EeziBleed. What I have to do is get someone to stand on the pedal while I rapidly open and shut each caliper nipple in turn. That always blasts an extra 'lump' of air out, and it is fine after that. I don't know whether it is the high pressure from the pedal with the nipple closed that unsticks air bubbles from the walls of the hydraulics lines, or the very high flow rate when the nipple is opened blasting them off the walls, or again the very high flow rate blasting the bubbles *down* the pipes and round the bends faster than thay can go back up again, or a combination of factors, but I need to do it on both my 73 roadster and 75 V8.
Paul Hunt 2

I had a similar problem with a 1970 GT. I did the same but opposite (if that makes sense) as Paul - I opened the nipple and had someone stamp on the pedal. I had tried power bleeding, engine running, different combinations left/right/fron/back, raising the front of the car, etc.

I wonder if the source of the problem is the three-way union, and whether bleeding all four nipples (or at least 2 front and 1 rear) at the same time would help?

Neil Lock

On a non-servo car, what is it about the engine running that would help bleeding the brakes? Is it the vibrations from the engine that cause more air to come out of the system?

Never thought to try it that way but next time I have to I'll be sure to try it.

Erick Vesterback

Thanks ever so much everyone.

I am a Triumph owner myself, and I am trying to help a fellow who is mechanically a neophyte. He has done most of the work himself to a quite high standard, but this problem has both him and me a bit puzzled.

I will definitly look at the rear adjustment, and follow the suggestions as to bleeding the system.

Thank you again, I hope to someday enjoy a vacation driving some sort of old sports car around your country.


Never underestimate the perversity of an inanimate object.

Re the running engine - *could* be vibration being better at shaking bubbles loose, but also could simply be higher hydraulic pressure - with the attached servo at least. I understand this gives significant assistance, and braking is pretty dire without it. The remote servo gives very little assistance and a negligible difference without it.
Paul Hunt 2


The standard remote servo on my 1970 car gives a lot of assistance - the difference between engine on an engine off is definitely noticeable.

Neil Lock

Maybe I'm particularly insensitive. Apart from the first tentative prod of the brakes after disconnecting it to see just how much difference it *did* make, rather than wait until I was rapidly approaching the back of someone else only to find it needed more effort to stop the car than I could deliver, and being surprised how little difference it made, I simply didn't notice in normal driving. As I say it was optional anyway at one point.
Paul Hunt 2

The vacuum canister is supposed to have the capacity to handle 2 or 3 stops with no engine vacuum. If you tried it just once, you probably still had reserve vacuum. Try pumping the brakes several times at rest. You will soon notice the pedal push becoming heavier.

As a practical matter my '67 gt with hi cf front pads does not require enough pedal pressure to even consider the need for a vacuum booster system. I guess the idea was to make the cars acceptable to the fair gender who were major purchasers of the B.


Barry Parkinson

Like I say I continued to drive it with it disconnected and still noticed very little difference.

"the fair gender who were major purchasers of the B."

Significant, possibly, surely not 'major' i.e. 'in the majority' ... unless you are possibly including Californian males ... :o)
Paul Hunt 2

In my experience, the dual piston master cylinder cars, post '67, required much more pedal pressure to stop than the single stage master cylinder cars. This is one of the major reasons American cars adopted the brake booster on '68 and later cars. Ray

Paul, That was a bit below the belt wans't it
Tim Boyle

I *could* defend it by saying that in the UK 'fair' can describe a persons hair colour, regardless of gender ...
Paul Hunt 2

It appears the macho brits are so insensitive they can't tell the difference between boosted and unboosted brakes. In California we are much more sensitive and aware and can perceive the difference.
Barry Parkinson

Did not see the year of this B. But if it has the brake switch mounted on the shroud for the pedals, you may want to look at if the switch is adjusted in to far.

If it is, it will prevent the piston from returning all the way and it ends up slightly blocking the fluid port in the MC. This will give a false feel to the pedal. When it sits, the pressure leaks out.
Bruce Cunha

Barry - well I never considered myself as 'Popeye on steriods' before ... :o)

Brake light switch (on the pedal cover) screwed in too far usually causes the brakes to come on or drag as the fluid warms up and expands. Will give a *higher* pedal, certainly won't cause the pedal to sink to the floor.
Paul Hunt 2

Try the pedal routine with the handbrake pulled full on. Let it set this way and if the pedal is okay you need to re-adjust the rear brakes. You may have oversized drums and the shoes are returning too far from the drums.
Sandy Sanders

Never found this problem on an MG but...I did find it on a lot(twenty plus)Toyotas,Nisans and Hondas durring the 90s
Hit the pedal and it went to the floor, hit it again and the pedal was fine. As I remember it would be fine until the car sat for a while and it would go to the floor again, once.... on and on until I replaced the master cylinder, problem fixed every time RIC

PS: as you might imagine I tried a lot of different fixes before I started replaceing what were very expencive items.
Ric Lloyd

Steve: you indicate you replaced the Master Cylinder as well as lines, flex hoses & shoes. Did you BENCH BLEED the Master before installation? This happens when air is trapped in the cylinder and bench bleeding prior to install gets the air out. You will have to do the bleed process all over again. Also, assuming you have someone pumping the brake while bleeding, be sure to shut the bleeder screw before the brake pedal is fully depressed, if you're not doing that already.

The same holds true for Slave cylinders on your clutch control as well.

It could be the new M/C unit is a defect.

Remember, new does not always mean it's good

Let us know how this problem solves out

Gary :>{D
79 mgb

This thread was discussed between 27/10/2006 and 09/11/2006

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