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MG MGB Technical - Brake fluid in foot well

Am looking for some advice. Have discovered brake fluid in foot well - '72 roadster. I removed the metal pushrod cover and found the rubber boot for the MC pushrod was filled with brake fluid. The paint all around has lifted significantly. Brakes so far working OK. Do I try repair kit or replace MC. Thanks.

Christian. Very good question and one that has no simple answer.

As the various systems have gotten older, some of them have been rebuilt several times, each time making the fit of the parts a little less exact. master cylinder bores can be either honed, using an abrasive hone similar to those used on brake cylinders, or flap sanded to clean up the bore. If, however, too much metal has to be removed, the seals will not fit and work correctly. If the bores are pitted, they cannot be honed because too much metal would have to be removed. In both of these cases, the bore is machined oversized and brass or stainless steel sleeve is fitted to restore the bore back to original size. This may be somewhat expensive, especially if you have the company install a rebuild kit at the same time. (Some companies only offer a resleeving service and will not install a rebuild kit.)

The 68-74 master cylinder is somewhat more difficult to rebuild than the later model MCs are.

For the above reasons, I have replaced all of my brake master cylinders rather than rebuild them. Four cars spanning a period of eight years. No problems to date and I know that when I next need to do a rebuild, I will be working on a part that has not been rebuilt an unknown number of times before.

What is best for you only you can answer.

Les Bengtson


Les is giving you the same good advice he gave me some time ago. I replaced the whole cylinder having fitted two rebuild kits, both of which failed in about 8 months. With my new cylinder I have had no issues so far and, as Les states, I know what the starting point is.


I tried a rebuild kit (managed to get one from the local Auto Palace). First thing was finding some circlip pliers that would fit. Cleaned, rebuilt, re painted, didn't work, maybe me. So for safety sake i bought a new one. No worries now.
G.L Lodge

It is funny that this topic has come up because several times now I have found what looks like brake fluid on my floor mat. This has only happen a couple of times. Now on my car (1973 MGB) the clutch and brake cylinders are almost new (three years old, 4,000 miles) and are not rebuilds. Plus, when I take the cover off in the engine compartment and look and feel, I have never been able to find any fluid around any of the connections or rods, or any wet areas, etc. Plus, my brake and clutch master cylinders are always full.

Where could this be coming from? Why doesn't it ahppen all the time? I have also felt and looked up under the dash and cannot find any drips or damp/wet areas.


Robert Browning

Thanks everyone. I have gone ahead and ordered a new MC. Hope it works. Robert - first thing would to be absolutely certain it is brake fluid and not water. If there is enough to collect you could apply it to a a peice of painted scrap metal - if it softens the paint it is obviously brake fluid. -Chris

Chris. There is no reason that it should not work fine. I have had no problems with the "new" MC on my 68GT over the last six years.

One of the big things for all of us to remember is that the Brits did not use a sealed system, as they did in the US. Thus, air, and moisture, can get into the system. A through flush of the brake system, every two years, will add to the life of the system. It will also make any future rebuilds significantly easier.

Robert. Water ingress, from a blocked tube below the air intake in front of the windscreen, is a common problem. The tube is poorly designed with a bulb and a mostly closed end. It fills up with junk, does not drain, and the water leaks from the cowl area. Eventually, this area becomes rusted through and more water collects. Examine the tube, clean it out well, and pull the grill and examine the area under it. That is the most common cause of water leaks into the passenger compartment. If it really is a brake fluid leak, some of the liquid can be picked up on a Q-Tip and spread over a painted piece of metal as Chris suggests. If it lifts the paint, it is brake fluid. If not, it is water.

Taking note of when it happens will also provide some additional indications as to the nature of the fluid.

Les Bengtson

Wow, seems like this is a common problem, counting myself as one who is currently suffering from this problem. Luckily, since I am doing this build up with racing in mind, my brakes and suspension are the first to get updated. Thanks for the info.
D. T. Barnes

Robert - I had the same problem with what I thought was brake fluid, but really turned out to be water in the drain tube getting backed up and leaking into the foot well. When I cleaned out the bulb it was surprising how much debris was in there - it comes from the opening just ahead of the windshield with the chrome trim on it. I took off the chrome trim, put a bit of rolled-up black vinyl screen in there and replaced the chrome trim. At car shows, no-one appears to have noticed the screen in there. Now debris collects on the screen, no more blocking up in the tube. Its easy to periodically pull the leaves and tree needles out of the screen even without removing the chrome trim piece.

Erick Vesterback

"One of the big things for all of us to remember is that the Brits did not use a sealed system, as they did in the US."

Are you sure about that, Les? I've heard of problems in America where the vent hole in the cap was blocked and as the fluid heated up and expanded it applied the brakes. This was on other than an MGB. I'd have said that *all* brake and clutch hydraulic systems have to have a vent above the level of the fluid for just this expansion and contraction.

Overfilling of the reservoir *will* cause overflowing when the fluid gets hot and expands. I can remember Minis in the 60s having a foam 'collar' round the filler cap on top of the reservoir. Occasionally I find fluid on the top of mine on the MGBs so have wrapped kitchen towel round the caps and secured with a plastic tie. One of the features of hydraulic fluid is that even when it falls on the floor it dries up and leaves no trace, so I'm nor surprised that there is no trace on vertical surfaces.

The bulb on the end of the drain tube is designed the way it is to prevent fumes from the engine compartment finding their way into the cabin via the heater, which could be dangerous. I don't really find it that surprising that they, like many other things on the MGB, need a little attention every 30 years or so! The tube can be rodded-out with a length of dowel and some clean water from above (so much better than getting a face-full of rotting gunge if attempting it from underneath. Probe vertically down from the right-hand edge of the right-hand square hole in the grille, tapping around, and you should be able to find the outlet. This is from a change in feel and sound from tapping on metal to tapping on rubber, and the dowel will go lower as well, Push down gently and it should come out of the bottom opening up the slit, which will allow clean water poured in from above to wash the gunge out. If the bottom of the air-box is clean the outlet is visible from above. Pictures at and click on 'Air-box'.
Paul Hunt 2

OK, let me update and also ask for suggestions. I have found out that the fluid in my foot well is brake fluid. I put down some towels and discovered that the fluid is running down the back of my brake pedal. This is why I have been having trouble finding it. So I pulled the pedal cover off under the hood - again - and this time I looked a lot closer and realized that the rubber boot has a small hole on the bottom side - which looks like it is suppose to be there. I pulled the boot off and it has brake fluid in/on it. Not much, just a little. So, is this normal? Should it be totally dry? I had added fluid about a month ago - an annual event - and I was wondering based on what Paul wrote, if I had over filled the reservoir, could/would it leak into this area?

The master cylinder was new about 3 years ago and only has about 4,000 miles on it.

Thanks? Suggestions? Should I mess with it now or wait till it gets worse - if it does get worse?

Robert Browning

Paul. From 1968 through 1980, the brake master cylinders had a rubber diaphragm which went between the two reservoirs and the cover. Each reservoir had a section of the diaphragm which was enlarged and, as the brake fluid level dropped, the section moved downwards to keep the seal intact. Much easier to visualize with one of the diaphragms in hand. Two mechanics tell me that the same system is in current use, but I only have direct experience with the 68-80 model cars. Older vehicles may well have had a non-sealed system. Have only owned two of them and that was so long ago that I do not remember what the single reservoir system consisted of. Yet another sign of getting old.

But, the basic system used on US cars from 68 onwards seems to have been the one using the rubber diaphragm between the housing of the reservoirs and the cap and it was a sealed system.

Robert. It sounds as if the rear seal is not sealing properly, allowing fluid to build up in the rear section of the bore which is normally dry. The only method of finding out what is actually happening is to pull the master cylinder, strip it down, clean and inspect it. If the seals look bad, a rebuild kit should be installed. Considering the leaking which is going on, I would have the rebuild kit in hand and install it even if the old seals looked to be functional.

Les Bengtson

I've rebuilt several Master Cylinders successfully and use a small honing devise that works wonders. If the cylinders are scored beyond safe repair (as mentioned above), then I've ordered rebuilt units from the major chains (e.g., NAPA, Autozone, etc.) at far less than what it costs for a new unit. MGB master cylinders from these outfits are in the 80 - 95 dollar range.
Joseph Maggiore

Joseph. One of the major problems with rebuilt parts today is that they seem to be of inconsistent quality. Several years ago, I went through five Auto Zone rebuilt starters in the space of a single year--one of which tested bad out of the box. The Auto Zone rebuilt distributor was so far out of spec it would have resulted in engine damage if installed--twice the proper mechanical advance it should have had. I went through four Auto Zone rebuilt air pumps recently on a warranty exchange. One lasted three months and the other three were incorrect parts and would not accept the pulley.

NAPA used to provide a better quality rebuild at a higher price. Several mechanic friends tell me that they are getting bad rebuilds from NAPA now, something that seems common in the industry.

The new master cylinders can be purchased from other parts companies other than Moss. I purchased my two at a significant savings through a local concern. About $200 each plus shipping.

Les Bengtson

This thread was discussed between 08/05/2007 and 14/05/2007

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