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MG MGB Technical - Brake Fluid Leak

My 72B sat undisturbed from late Oct until yesterday (11 Mar) in an unheated Minnesota garage.

The brakes were fine when I parked it.

Yesterday, I started the car up for the 1st time since last Oct. I let it idle for awhile and drove it slowly out of the garage into the 60 degree sunshine. After letting the car warm up a bit more, and after looking things over I drove it out of the drive way slowly approaching the stop sign 100' feet from driveway.

To my surprise I had no brakes.

Returned car to driveway and discovered the master cylinder was empty, both front and rear compartments.

I have found no sign of leakage on ledge under master cylinder. I have found no sign of leakage around pedals or floor mat (left side drive).

I did notice large staining on the cardboard upon which I parked the car for 5 months.

I have not tried pumping pedal while looking at lines for leak, but will do that soon.

Any ideas what went wrong?

Oh, master cylinder was replaced in 2008, new - now with < 5000 miles on it. Using synthetic brake fluid.
R.W Anderson

Your brake system is leaking from some place and the fluid is going somewhere. You just have not been able to determine where it has gone. Perhaps onto the carpet on the driver's side floor.

Refill the system, bleed the system, then begin your hunt for leaks. Newspaper, taped to the floor of the garage and under the brake and clutch pedals on the driver's side floor, should show up drips/leaks.

Les Bengtson

Always crazy when this happens. I might have said is the car was in an unheated garage, maybe when all got very cold that a rubber seal might have shrunk? but then, if the system was sealed, how could fluid escape altogether? Mystery. As les said re-fill and check?When this happened to me onec, I know what I did! I had not tightened a nut securely, that's how it was lost! What a ding-dong. Mike
J.M. Doust

My money is on the brake pressure failure switch. It is the five way valve thing on the fender. See if you have a bunch of fluid running down the fender from that location.

And all brake fluid is synthetic. You can't go to the brake fluid well and draw some ;)


Dave Braun

An Update: So far!

As stated originally, there is no sign of leakage inside the car or under the hood.

Cardboard under the car indicates the fluid leaked out in the general area below the master cylinder, and it leaked out some time ago (between parking in Oct and moving in March), as all that is left is the stain, and cardboard is essentially dry, but I have a 24 inch circular stain.

Yes, car is stored in unheated Minnesota sub zero garage, as is my regular car. Never had a temperature related problem before.

I have refilled the divided reservoir in brake master cylinder and nothing has leaked out since. I have not had time to bleed the system yet, but pedal is pull and strong with no downward pressure loss.

I sprayed brake cleaner everywhere under hood and wiped area dry and clean. With someone pumping brake I can get no leakage anywhere.

As to Dave B's suggestion to inspect brake pressure failure switch... This area has considerable paint damage indicating past leakeage. But pumping brake had does not cause a current leak.

As the brake lines quickly move away from the master cylinder area, the only parts left are the master and the pressure switch. Which both look spotless at the moment.

I will bleed system next, to perhaps increase pressure more to see if I can cause a leak. Otherwise I will be stumped as to how it all leaked out over winter.

More later.
R.W Anderson

Maybe some-one crept into the garage, thought they were syphoning off some fuel, and got a mouthful of brakefluid instead. Sorry, stupid idea! Mike
J.M. Doust

Cold weather may cause the seals in the MC to shrink and leak. My B does this almost every year and I have to top the MC off, bleed the brakes and they are good for the summer. I also know that my MC is getting to the point of replacement, yours sounds a bit new for that. Check the carpet or mat in the drivers footwell. The fluid will evaporate a bit but it will damage the carpet or mat.
J Mancone

Be aware that the brake failure switch is not a seal. The seals are internal, on the left hand side of the master cylinder (left as when seated in the driver's seat). There are rubber O rings which keep the brake fluid from leaving the body of the master cylinder and moving into the area of the failure switch. If these O rings deteriorate, fluid is lost through the opening and air is sucked into the system. Most of the "rebuilds" I have seen performed did not address this section of the master cylinder, only the main bore. This, however, should not be a problem with a relatively new master cylinder. Mine have been in place for four to eleven years without any problems.

Les Bengtson

I have a 1979 MGB and had the same issue. It turned out, the seal from the M/C to the vacuum booster failed and the fluid was being sucked through the vacuum booster and was being burned in the comustion chamber of the engine along with the fuel. Hence, no puddle, no wet surfaces, just no brake fluid. Scary when you discover no brakes coming off the the Interstate highway.

So in the end, replace the M/C and that should solve it.



79 MGB
gary hansen

A bit drastic, since the fluid was lost while the vehicle wasn't being used, and the cardboard under the car was stained. As a 72 it wouldn't have a servo on twin-circuit brakes anyway, would it? No mention of a servo, and I don't see how it would have emptied the front reservoir anyway.
PaulH Solihull

In further follow-up, PaulH is correct I do not have a servo. And yes it is a twin comparment MC reservoir. Both sides of MC were empty.

I've since checked wheel cylinders, and no sign of leakage.

I've placed brown paper bag beneath lines and brake failure switch to see if staining occurs. So far nothing.

And to repeat, no sign of leakage at MC or inside on pedals or floor. And MC is rebuilt from the outfit in Wisc I think, which I thought went through the whole cylinder. I'll have to dig out receipts to make sure from whom and if new or rebuilt.

Since refilling reservoir, no sign of leakage.

I have not bleed system yet. Have not driven car since unexplained leak either.

More later.

R.W Anderson

Further update:
The paper bag I placed under the pressure switch is starting to stain, showing a very slow leak of brake fluid coming out of one of the lines in the bottom of the pressure switch, not an end - the bottom.

I've used penetrating fluid to loose the nuts on the line to try and tighten the lines but nothing.

Repair kits are only for the end of the switch, so I may try to remove lower lines to see if end is worn and leaking.

Not sure what else to do with a leaking switch line.
R.W Anderson

This thread was discussed between 12/03/2012 and 27/03/2012

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