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MG MGB Technical - Brake headache

Hi all,

I was braking for an upcoming set of traffic lights and the brakes were working until suddenly the pedal sank into the floor. It wasn't an instant snap to the floor. There was still "some" pressure in the system as the pedal sank slowly and brakes stopped working as soon as it hit the floor. I released the pedal and applied the brakes again it was good. It was a bit dicey, but I was almost at work so I completed the drive carefully with it "failing" at each stop.
I checked for leaks during my lunch break and I couldn't find anything. To my surprise everything was okay. Drove around the car park a few times and the brakes didn't fail once. I drove home okay until the last few stops near home when it started playing up again.

Thinking it looks like a seal, but not finding any leaks and the master cylinder was still filled with fluid, I grabbed my Haynes manual and decided to do a thorough job bleeding the system.

Using a one-man system. Not the gunsons with pressure from a spare tyre, just a simple unit with a valve on the end to stop the fluid and/or air returning with each stroke. I adjusted the rear brakes first. Handbrake was nice and firm. Then I followed the Haynes instructions step-by-step; including the part about the pressure failure switch.

Plenty of air came out of the system and it took 3 bottles of fluid before I got steady air-free streams coming from all wheels. I finally ran a long piece of tube from each wheel all the way to the master cylinder and pumped the pedal watching the tube for any contaminates to drain off into a spare container. I trapped only a few minor specs of whatever (probably the seal I'm still thinking about).

Still no brakes. Even worse than before when it was braking for a second and then failing then coming good after a single pump. It just fails straight away now.
I can hear a very faint air-sucking noise when I pump the pedal fast, so clearly I have some sort of leak. I don't have an assistant to help me find the exact location, but it sounds like under the hood somewhere. The master cylinder looks fine, but what's inside?
I pump for ages and it doesn't change. The fluid level also doesn't change, so it isn't going anywhere. If I take a quick "sample" bleed from any of the wheels, it's nice and clean fluid with no bubbles.

I left it overnight and started from the beginning in case I did something wrong. Bled the whole system again and this time there wasn't any air coming out and no contaminants. Nice clean fluid the whole time.

So is it a seal like a first thought?
Where do I look? There are no leaks around the bleed nipples or any of the hoses and connections. I have at least some pressure in the system or the bleeding process wouldn't have pushed fluid through to each wheel, would it?
D O'Brien

I would suspect the master cylinder seals or the seals in the remote servo, assuming you have one.
Mike Howlett

Almost Certainly Master cylinder, the fact that it pumped was just that you removed resistance, so the piston itself rather than the seals (or their remains) was pushing fluid around. Now that you are trying to force against the brake cylinders, the easiest path for the fluid is past the seals.
Pat Gregory

If you weren't losing any fluid than it is the master pressure seal leaking back into the reservoir. Servo, slave or calliper seal problems would be losing fluid.
PaulH Solihull

A leaky pressure fault valve surprised me with a failure similar to the one you experienced, except by the time I found it I didn't have any brake fluid. The engine bay was dirty enough I didn't notice the problem until the brakes failed. Sure took off a lot of paint though...

Dave Braun

It sounds like PaulH is correct.
I'm looking at a diagram on which shows the only diagram I've seen which has the rectangular reservoir the same as mine. All other diagrams I've seen (including the Haynes manual) show a sloped reservoir.

Anyway, how do I get at this master pressure seal? In the diagram, the sub-section labeled "4" shows all the seals. Which one?

Also, I notice a little circlip in that section and I vaguely remember my old mechanic spending half a day inserting that when he replaced my master cylinder a while back. Should I give it back to him and wish him luck or is there a trick to doing it that I can do myself? Getting the car to the mechanic now I have no brakes at all is a bit difficult.
D O'Brien

The diagram shows item 4 to be a complete kit for the master cylinder. I have never replaced a single seal in a master and I (and I am sure others would agree) would only replace all seals at the same time. In fact I can't remember being given the choice of a single seal, only full kits. At 10 GBP I would say it is a bargain.

Tony Oliver

If any seal needs replacing I will definitely be replacing all of them. I'm just interested in where I should be looking exactly to determine if it is indeed the master seal or if I'm barking up the wrong telegraph pole.
D O'Brien

Sorry about that Chief. I misread you. I will happily defer to the other experts on this BBS but if it were my car I would replace every seal in every part of the system. If one seal has disintegrated, as you seem to suspect, than how far behind it are the others. I have found in the past that it is cheaper to buy complete rear slaves than it is to buy the kits.

I know it's a bit of extra expense, but it's cheaper than running up the rear end of an Aston DB9! I have never really cared if any of my cars have been the fastest in the world, but I sure like them to stop quickly.

I have had many dealings with this mob in Sydney over the last 4 to 5 years. This is a link to the brake section of their online shop.

No connection to them except as a very happy customer.

Tony Oliver

If there is no fluid loss, then MC seal is the only possible fault.

If you have NO brakes, then at least two of the four seals are bad. Starting from the closed end of the cylinder, #1 & 3 are the primary pressure seals, each letting the pressure out from one circuit. Or it could be #1 & 2, with #2 letting the pressure from #3 out. #4 seal lets fluid run out the end of the cylinder, into the booster.

You should be able to see the fluid coming back into the MC reservoir when somebody steps on the brakes. If the fluid is too nasty to see through, that'll be your trouble. Typically, if you jam the pedal down fast and hard, you will get brakes, but slow/easy application lets it bypass.

FR Millmore

Because the MGB system is split front and rear if the front fails the rear alone will give very little retardation by comparison, and it would be tempting to think you have no brakes. However even that shouldn't allow the pedal to go to the floor, although it will go further than normal. If it did sink to the floor, then as FRM says both circuits must have failed, although for both to happen at the same time would be a helluva a coincidence. Maybe the rear circuit has been bad all along and you have just got used to it, but then the brake imbalance light should have been on ... if it is working.
PaulH Solihull

No apology necessary Tony. I drove my car with dodgy brakes so one might assume I would do other silly things like not replace all the seals for the sake of a few dollars. I might be silly, but not that cheap.
I'm already a regular, happy customer of Heritage, but thanks for the link anyway. I didn't know they had upgraded their website with a proper online shop.

I left everything spotless and clean last night and put paper towels under everything to see if a leak appeared anywhere. If anything good from this whole exercise, there are parts of my car that have probably not been this clean since it left the factory.
Anyway, definitely no leaks. No loss of fluid. I just tried the slow vs. fast pedal test and it does seem to have a bit more resistance for a brief moment when I press fast, but still no brakes.

I will be pulling the master cylinder out this weekend and replacing all the seals.

Thanks all for the help and advice.
D O'Brien

I have replaced countless master cylinders where both circuits have failed at the same time. When one fails, the amount of pressure required from the remaining half of the braking circuit, often overwhelms the seals and causes a total loss of brakes. This is the main reason tandem master cylinders aren't allowed in racing applications. Two separate master cylinders, connected by a mechanical bar, are required. RAY
rjm RAY

My money is on the master cylinder seals. And yes consider replacing all. I still remember, as a young driver in my mini minor 1965, coming down a steep hill, when the brakes totally failed. So after my whole life rushed past my eyes, ( it was short, so it was over quick!) I had to make the decision; carry on to the bottom of the hill and negotiate a left turn in the road, or turn now into the hedge ( British country road.)..... I will leave you with that cliff hanger. Must have been ok, I am still here!!! Mike
J.M. Doust

You only need to monitor the master to see if you have leaks. If the level drops you have, if it doesn't you haven't. Brake fluid evaporates, and can run down the brake pedal to be 'lost' in the carpet, so external evidence of a leak may not be immediately apparent.

"or turn now into the hedge (British country road)" - two yokels on a tractor came out of a field gate in a country lane right in front of a chap driving down the road, who just managed to avoid the tractor and the hedge by going through the gate. One yokel looked at the other and said "We came out o' there just in toime".
PaulH Solihull

Well I've spent all weekend just about and it's still giving me nightmares.

I got the seal kit. I was going to buy a complete master cylinder and make the job easy but they didn't have it in stock.

I followed the Haynes manual (1962 thru to 1980 Roadster and GT Coupe) chapter 9 section 20 "Tandem master cylinder, later type with integral PDWA - removal, dismantling, reassembling and refitting"
This appears to be the closest to what I have in my 1978 Roadster (UK model), although the diagram shows a sloping reservoir and it talks about different seals in the primary port (a seal adaptor, etc). Mine has a rectangular reservoir and both primary and secondary inlet ports are the same and the seals that came out were the same for each.

Anyway, I dismantled everything except the pressure differential piston. It says to "extract the pressure differential piston by air pressure through the primary outlet port."
I've tried everything I can and it won't budge.

So I carried on and did all the seals on the primary and secondary piston. Again the manual says one of the springs is shorter (mine are same length) but otherwise the shape and design of my pistons is the same as the manual.

I made sure I had the lip of each seal facing the correct direction according to the manual. Replacing those 4 seals seemed pretty straight forward and I can't see where I could have done anything wrong.

The seals on the inlet ports look a little different to the ones that came out. The new ones look like the manual (except both are the same and I don't see how/why an adaptor and o-ring would be used in the primary inlet).

Now I've reinstalled it and it isn't getting fluid from the reservoir to the master cylinder. A tiny bit gets through, but not enough to bleed the system.

Aside from the PDWA I couldn't remove, what could I have done wrong?
D O'Brien

You did note that the orientation of the seals on the front piston is different to that on the rear? But even then, with the push-rod fully released both seals should clear the passages between the reservoir and the two pressure chambers. How are you determining fluid isn't getting through those passages, or is it that the piston isn't pressurising it? What bleeding method are you using, a continuous pressure system like the Gunson's would be preferable.
PaulH Solihull

I'm using an Ezi-bleed system which is basically a short tube with a valve on the end. Similar to the Gunson's but without the pressure being provided by your spare tyre. I have to pump the pedal but I don't need to open/close the nipple in between each pump.

Yes I noted the different orientation. I thought that's where I might have gone wrong so I pulled it all apart and did it again yesterday. It was right.

The fluid didn't seem to be getting through the passages because the level wasn't dropping in the reservoir no matter how long I pumped the pedal.

On first attempt I found it difficult to get the reservoir and body to match up. It seemed as though the two ports on the reservoir couldn't fit inside the diameter of the seals once they were squashed into place on the cylinder body. They were lining up, but not slipping inside the hole.

After pulling it apart a second time yesterday, I tried with the inlet port seals on the reservoir first, then placed it on the cylinder body. I couldn't get the seals to seat completely into the body, although the whole thing sits a lot neater than it did the other way around.

With fingers crossed I put it back together. I still had the front wheels off and although I know you start at the rear wheels first I gave a quick shot at bleeding to the furthest front wheel.
Voila. Something is working. Fluid is getting through easily now and I was able to bleed both wheels.
I'll swap the jack and axle stands and do the rear wheels tonight and see if it's working or not.

Aside from re-doing everything and not completely seating the inlet port seals, the only thing I did different was pump extremely slowly for a while to try and bleed through to the first wheel. I'm sure I didn't pump too fast on my other attempts, but could that be all the problem was (not the original problem of course, that WAS a seal)?
D O'Brien

This is where the master part of the Gunsons EeziBleed system helps more than the caliper/slave cylinder part. If the system is empty then you may simply be squashing and expanding the air with each pedal pump. Using the pressure bottle on the master i.e. a continuous flow system is much easier and more effective. The only other way is perhaps to hold the pedal down for much longer than you might be, but the pressure bottle is preferable. You *could* do an on-car bench-bleed to check the master, by removing the pipes and connecting a tube to each port but directed back into the reservoir (cap removed). If that doesn't give good fluid flow from both tubes then you know something is wrong with the master.
PaulH Solihull

Maybe it was the extra slow pumping, or maybe I was holding the pedal down longer. Either way it seems whatever I did last time has worked.

I got back to it last night and immediately swapped the jack/stands around so I could attempt to bleed the rear wheels. Fluid was coming through very easily with each pedal pump, albeit with lots of bubbles. Eventually it came through clean and free of bubbles.
I moved on to do the front again and everything continued to go smoothly.
Just to be sure, I ran my tube all the way from each wheel back to the reservoir and bled each of them slowly for about 15 minutes (my right leg muscles are twice the size of the left leg now).

I've put everything back together except the front wheels are still off. Pedal feels good. I'll put the wheels back on tonight and take it for a careful test drive.

For next time, I've ordered one of those Gunsons systems. I thought the flowback valve was the important part of a one-man system, but I can see how the pressurized system is a major advantage now.

Thanks again for all the advice.
D O'Brien


Thanks again. I'm taking off the personalized "IN2HEG" registration plates now. Brakes are all good. :)
D O'Brien

Just a tip for next time, when bleeding brakes on a cylinder that is not NEW only push the pedal half way down (approx) then up don't go to the floor, (think about it, if the poor old seals travel down to the bottom of the cylinder, where no seal has gone before, over all the crap, it rips the seals.
rj clark

Hmmm, I think I'd rather rip the seals during bleeding than when driving.
PaulH Solihull

This thread was discussed between 05/10/2011 and 13/10/2011

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