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MG MGB Technical - strange problem with brakes

Last night i was driving my 1977 bgt, I did approx 40 miles when it seemed to lose power slightly. When the power came back after about 30 seconds, I found that my brake pedal went nearly to the floor! This was a bit alarming. The pedal did come back to more like normal after pumping it, but the brakes still dont feel right. They stop the car fine, but i am lacking confidence in them now.

Has anyone else experienced this, or any ideas on what to check?

Thanks
John
John

First guess I'd say is failure in the servo which has caused a vacuum leak into the inlet (hence the loss of power) and maybe loss of brake fluid (either into the engine or into the servo) hence loss of pedal.

CHECK IT carefully before using again - might be lethal. Visual on servo and fluid level may help.

Regards,

Andrew
Andrew

Andrew,

Thanks for ideas, I've had a quick check and the fluid level hasn't dropped, there are no leaks anywhere else either.

The pedal doesn't feel too great, but if I pump the pedal it does "harden up" as if the servo holds pressure.

One aside, when on the motorway, the car did pull to the left under braking (I thought that was strange as it usually pulls up straight) maybe a binding brake would have boiled the fluid, and as it cooled, i got brakes back? and now that the fluid has cooled, it isn't as good as it should be due to water absorbtion? Just another idea?

I guess, that I will get the midget out of the garage tonight, ready for work in the morning!

John
John Collins

Sounds like the brakes were binding, eventually they overheated and boiled the fluid, this pushed the fluid back up the pipes and released the brake.

This happened on a S&M trip in london due to the pedals binding a little to someone who narrowly avoided hitting me.
Will Munns

John
The reason I looked on this web tonight was for almost the same reason that you experienced.
The servo on my car is almost new and there are no visible leaks. There was no loss of power and the pedal almost to the floor occurred after only a few miles of gentle motoring - pumping brought the pedal pressure right back up. The car had been sitting for a few days. No visible brake leaks. Fluid level was down only slightly.I have a recollection of seeing something on this message board - maybe a couple or more years ago and that it was sssociated with a leak in the servo diaphragm ( could have seen it in a magazine)
I'll be following this thread with interest

Mike
Mike Armstrong

John - If the problem is due to the fluid boiling, you will need to bleed the system to remove the old fluid and gas bubbles. It's a good idea to do this every year or two anyway.

If one of the brakes overheated, an examination of the rotors and drums should reveal metal discolored blue and/or yellow. Also, you may see evidence of melted or burned grease. In this case, you should consider replacing or rebuilding the calipers or wheel cylinders on both sides of the car.

If there are no signs of overheating or fluid leaking at the wheels, it may be time to overhaul the master cylinder. As always when working on brakes, check the condition of the hoses and replace any that look cracked, aged or damaged.

Brakes are a critical safety item. It would be best not to drive the car until this is sorted out. -G.
Glenn G

Thanks for the ideas, I'm not using the car at the minute, the midget and the mx5 are in use instead.

I will strip all four brakes and drain the fluid next weekend over the easter break, I'm guessing that its the left front as it pulled to the left on braking.

I will post my findings.
Cheers
john
John Collins

John
There are some references under "Brake Servo" in the archives.
I still think that you should think of failure of some part of this - there is apparently a rubber seal which -if it leaks- can give you loss of pedal pressure but pump action of the pedal brings the pressure up.It can be replaces at a cost - but cheaper than a whole servo.
The loss of power is also a symptom.

Mike
Mike Armstrong

John
Are you dual circuit with integral servo? A 77 should be. If so a pressure loss to the front should have moved the shuttle and lit the brake warning lamp.
Check the bulb by starting the car with the handbrake off and it should glow. Check the condition of the servo hose and clean the non-return valve which is the cream coloured 'union' where the hose meets the servo.
The dual circuit m/cyl reservoir can deceive as it's split internally into two, so look again. That loss of power sounds more like ingesting brake fluid to me.

If it's single circuit brakes then check the none-return valve and also unclip the air filter on the servo and make sure it's not blocked. It's the upside down plant pot shaped cover in the middle of the cream disc.
Dave Wellings

"Check the bulb by starting the car with the handbrake off and it should glow"

Even easier does it glow with the handbrake on? But both of these tests only the warning light and part of its wiring, there could still be a problem back at the shuttle switch which prevents that lighting it. FWIW only US spec cars have the shuttle switch according to my diagrams.
Paul Hunt

This thread was discussed between 10/03/2005 and 18/03/2005

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