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MG MGB Technical - Break fluid leaking from servo

I have a 1977 B and it looks as though there is brake fluid leaking from the underside of the brake servo. The underside is damp and there is a small pool on the shelf of the inner fender. I've done the majority of work myself on the car but this is foreign territory for me. Never have done anything with the servo unit. What might be the suspected problem, how costly might it be and is it something the average do it yourself guy can tackle? Just started looking at the manuals and thought I'd make this post in hopes that someone can identify the problem and give some direction.
JCH Hibbard

JCH the fluid is comeing from the master cylinder, you will see it as soon as you loosen the M/C it will drip fluid at the split, let us know. Ric
RIC LLOYD

Thanks RIC, I took a mirror and looked underneath, the bottom of the servo is all wet but it does look as though it originates from where the MC connects as you said. Haven't done anything with the servo or MC before so a couple of stupid questions. What does the servo actually do? Will it need to come off and is there any brake fluid in it? Secondly I assume that the seals in the MC are going bad. It looks pretty straight forward getting it off. Any experience with using rebuilding the MC with the kits offered or is new the way to go?
JCH Hibbard

JCH,

Rebuilding the MC will cost about $20 and replacing will cost about $175 - $200. If you rebuild, you will also have to spend about $20 for a brake cylinder hone, and you need a variable speed drill.

The decision to rebuild or replace depends upon your money vs time consideration and upon your mechanical skill level. If you have never rebuilt hydraulics before, you will have a bit of a learning curve.

It is always possible that rebuilding can turn out to be a temporary fix. I would take a guess that, over the years, I've seen rebuilding work in the long term about 75 or 80 percent of the time.

Charley
C R Huff

Unless cash is a problem go with new your first time out. once you have the leaker in your hand you can play with it, rebuld it, and if you are happy about the way it comes out, sell it on E-bay. Ric
RIC LLOYD

I don't think I'd want to buy a MC that was someone's first rebuild experiment.

Charley
C R Huff

JC. The servo is your "power brake" mechanism, reducing the amount of force necessary to apply the brakes. It does not contain brake fluid.

To remove the master cylinder, drain as much fluid out of it as you can. A turkey baster helps with this. Loosen the brake lines attached to the master cylinder using a line wrench. Remove the two nuts holding the master cylinder to the servo unit. Remove the master cylinder forwards. Installation is the reverse.

You can have someone like John Twist, of University Motors Ltd, rebuild your master cylinder. But, considering the age of these vehicles and their often unknown history, I have elected to install new master cylinders on my cars. My latest ones were obtained, at a significant savings over previous ones, through a dealer who purchases from North West Imports.

Rebuilding this model is fairly easy. Except for the section used for the brake warning light system which must also be rebuilt as part of a master cylinder rebuild. That is fairly easy if everything comes apart properly. They do not always do so and, then, it becomes interesting. When we purchased my daughter's car, the master cylinder had been rebuilt four times in four years by the MG Shoppe. But, they had never rebuilt the brake warning light section which was where the problem was.

If necessary, the servo unit can be rebuilt. More of a job than the master cylinder to remove and you need to make special tooling to do the job properly. (Illustrations contained in the factory workshop manual.) I would think, however, that if the servo unit is currently functioning correctly it has not been damaged by the leaking fluid. But, I would correct the problem soon.

Les
Les Bengtson

The in-line servo (as opposed to the remote servo which does have fluid in it) is easy to renovate and a kit can be purchased for not a lot of money. As others have said there is no brake fluid in it. It is a purely mechanical device. I did mine with no trouble at all.
Mike Howlett

I'm thinking by the sounds of things that new is the way to go. Will I be able to use the current pressure failure swith of the old MC, or does that come with the new MC?
JCH Hibbard

JC. The new master cylinders come without a switch installed. You have to either use your old switch, purchase a new one, or plug the hole.

If you plug the hole, you deactivate the brake failure warning system. My experience, on both MGs and other cars, is that the failure light comes on as you are going "Oh, Sh*t!". Hence, I am not worried about deactivating the feature. When the hole is plugged with a bolt, leakage around the rubber O rings in the warning mechanism does not cause air to get into the brake lines, nor does it allow fluid to drip onto the inner fender. The switch does not physically seal the opening, allowing any fluid which gets into that section to drip out and air to get into the main bore of the master cylinder. But, whether to keep the warning system functional or not is a personal decision.

Les
Les Bengtson

Les thanks for the advice. I have taken off the cylinder and the switch is apparently broken at the threads and will need to be replaced. Also the electrical connector that goes on the switch is corroded. I will need to replace the switch as well as the connector. However, I do not see anything listed at Moss or VB for the connector, any ideas where to get one? Out of curiousity what size bolt plugs the hole if I choose to go without the switch and does it require any sealer?
JCH Hibbard

Additionally, can anyone explain how to bleed the brakes with a bolt in place of the switch. Manual says back of the PDS 3 1/2 turns when bleeding. If using a bolt with a sealer does it need to be turned out as well when bleeding?
JCH Hibbard

I'd guess that procedure is more to do with centralising the imbalance switch after bleeding than anything to do with the actual bleeding itself.

I've often wondered what pedal-bleeding a dual circuit system is like, one would imagine that the closed circuit would prevent the pedal moving very far, which would affect the bleeding of the open circuit. If you weren't already going to you may need to consider continuous flow bleeding from a gunson's EeziBleed or similar.
Paul Hunt

I've often wondered about bleeding dual circuit systems. All the dual circuit cars I've had show the bleeding procedure as open one wheel nipple and press pedal to floor ........ and it does go to the floor !!

Why is the second circuit not holding the pedal? I sent the question to Practical Classics several years back, but no answer.
I am converting my MGB to dual circuits (as I prefer the servo arrangement, so I'll report back when done !

Regards,

Andrew
Andrew Walton

I would like to replace the switch but may have to go the bolt route. I can not seem to locate the electrical connector that attaches to the switch. Mine is corroded and will not make the connection so otherwise unless I can find one the switch is usless.
JCH Hibbard

JC. The bolt does not have to be removed when bleeding the system. The reason the switch must be retracted is because it is a normally on switch with a spring pushing a plunger upwards. The plunger bears against a rod inside the balance/brake warning portion of the master cylinder (on the left hand side). This rod has two cut outs on it, one fore of the switch and one aft of the switch. If there is an imbalance, either when bleeding the brakes or through failure of one of the systems, the rod is pushed towards the weaker side and the plunger on the brake failure warning switch moves upwards, both turning on the light on the dash and locking the rod in place. You have to lower the switch to turn off the light after any problem is repaired.

When bleeding a dual master cylinder, it is common that the pedal will not work in the same fashion as when you are depressing it with fully functional brakes. The system is designed to isolate the front and back brake systems from each other, allowing you to retain some small braking capability. The key word is small. But, it was a federally mandated "safety change" and does give you some slight braking capability rather than the none which is a result of system failure with a single master cylinder. All brake systems are bleed the same way regardless of whether it is a dual or single master cylinder.

One note of interest is that the furthest away wheel cylinder should be bled first. In the case of the MGB, left hand drive models, this is the left rear brake wheel cylinder. Check out the routing to see why.

Les
Les Bengtson

Thanks Les final question, size bolt and what type of sealent or JB Weld?
JCH Hibbard

JC. Thread size is 3/8"-24 UNF from my notes. I also just checked a switch to confirm that, so my notes are accurate. The threaded portion of the switch is about .358" long. (One quarter inch is .250" and three eights inch is .375", so you want to use either a 1/4" long bolt or a 3/8" long bolt with a couple of washers.)

As to joining compound, you do not need it with a good master cylinder--there is no brake fluid present near the switch unless the rubber "O" rings on either side have deteriorated. That was what happened on my daughters 77B daily driver. In that case, I cleaned the threads with a Q tip and brake cleaner, then secured the bolt in place with some hard setting Form A Gasket. It lasted quite well until I could afford to replace the master cylinder with a new one. Probably about two years with the bolt in place and no problems noted. I replaced the master cylinder when I rebuilt the car's engine prior to her going off to college because I would not be there to inspect/perform preventive maintenance on a regular basis.

I would use something like non-hardening Form A Gasket with a new master cylinder. That would make it easier to remove should it ever need to be removed.

Les
Les Bengtson

I just finished installing a new master cylinder and I have a question for those of you that may have used the Moss aftermarket cylinder for 169.95 rather than the O.E. for 249.95. I purchased the aftermarket and all went together smoothly enough but I kept getting leaks down the inner fender liner from somewhere??? The short story is that the fluid resevoir was not tightened all the way down and the end plug to the pressure differential piston was also not tight.All looked okay out of the box nothing felt loose but the connections were not as tight as they should have been. Once I snugged everything down no more leaks! But I am a little concerned now about installing the aftermarket and am wondering if any of you have it in your car and if it is proving to be reliable.
JCH Hibbard

JC. I do not order from Moss, after my con rod failure, unless no one else has the parts. Hence, I am not aware of what they list as an "after market" part. Who makes this part? I have used two of the TMW? master cylinders for the later model rubber bumper cars with good success. No problems out of the box (but, I do check the tightness of the reservoir screws and the cover screw for the balance mechanism because I have seen them loose before) and in two-five years of service on the roads. The ones I purchased came through a local dealer who purchased them from Northwest Imports.

Les
Les Bengtson

Les this was manufactured by Uroparts. Guess I should have thought to check for tightness ahead of time. By the way I used your 3/8 24 unf bolt idea with a little non hardening form a gasket all went well and no problem bleeding as you said, thanks.
JCH Hibbard

This thread was discussed between 06/02/2009 and 02/03/2009

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