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MG MGB Technical - Master Cylinder 72B

Spring brought with it what I thought was condensation on the drivers floor mat. I started B up after it sat for winter and it fired right up so off I went for a short spin. I did notice what I thought was a soft brake pedal and my shoe sure slipped on the water on the floor mat.

I returned home to take out the floor mat to dry and realized it wasn't water it was brake fluid.

My master cylinder appears to have sprung a leak over the winter. The clutch reservoir was full, but brake was empty.

I filled reservoir up for now and pedal is back up, but I know it will be parked until I can rebuild the master cylinder.

I see new master cylinders are $200. Rebuilt kit (6 or 7 seals) is $20.

I anticipate the worst part to be getting the brake lines off of the cylinder.

How bad and time consuming is a rebuild on the master cylinder, and are they typically sucessful?

I'm also wondering why a leak developed while setting over the winter.

One point though and I don't think they are connected is that last fall well before putting car away for winter, I flushed out the brake lines after replacing front wheel hoses and replaced with synthetic brake fluid. I'm hoping the leak and the synthetic aren't related.

Thanks for any assistance.
R.W Anderson

What type synthetic fluid, are you talking silicone fluid or something like Valvoline SynPower? Synthetic fluid shouldn't hurt the system. Silicone fluid in an old system may cause problems. Some people rebuild those cylinders but they aren't the easiest of cylinders to rebuild.

Clifton
Clifton Gordon

I used the Valvoline SynPower (synthetic, not silicone).

I see new is $200 for a master cyclinder and one local recommendation is to take it in, have it honed (sp?) and install a stainless sleeve and then rebuild. This appears to be $90 to hone and install sleve + $30 for kit or $175 if they do it all.

How necessary is it to do the stainless sleeve?

Does one ever need anything other than the rubber parts in the rebuild kit?
R.W Anderson

The necessity of the sleeve is dictated by the condition of the bore.
Is yours pitted?
Kimberly

The cylinder has a step, so there are two inside diameters. If the front part hes pits it's difficult or impossible to hone that section with an ordinary brake hone. There is a nylon guide bearing that is usually difficult to remove and you have to destroy it to grt it out. I believe a new bearing comes with the kit. Behind the spring retainer is a spiralox ring that may be badly rusted, it was not in the kit I purchased. Usually the kit contents are all you need for rebuild.

I rebuilt one 10 years ago, although the cylinder visually looked good (I honed the small section of the bore) it was leaking again in six months. After that I purchased a new cylinder.

Clifton
Clifton Gordon

I rebuilt mine in 1991 including honing both sections. I am using silicone fluid and have no problem to date. Be very carefull of the chrome surface of the rear plunger, if the chrome is damaged, it will leak. It is easily damaged during disassembly.
John H

R.W.-
These are your brakes. Spring for a new MC. I'm cheap, but when it came time for a MC on my midget a few years ago, and my GT a few weeks ago, I bit the bullet and bought new. taking the lines off the cylinder was easy, putting them on was harder.

Don
don g

Rebuilt master cylinders are sold by the thousands every day at you local parts house for all types of cars and trucks. A new master is no better then a properly rebuilt one. If the bore in your cylinder is smooth with no pits or score markes it seems a waste of money to spend $200 instead of $20 as all that has happened is the rubber parts are past there usefull life and a new unit will contain the very same parts.
John H

RW,

Getting the lines off will probably not be a problem if you have a proper line wrench, unless they are so rusty that they need to be replaced anyway. Go shoot your lines asap with your favorite old nut lube. You don't want the tube to be frozen to the nut, as that is normally the only problem with the lines.

Generally, I consider having a cylinder sleeved as worthwhile only if it is not possible to buy new, but I don't think it is cost effective for one that is readily available.

I also don't think it is cost effective to pay someone to rebuild yours when a new one is available.

If you have the big round cap style cylinder type, I rebuilt one of these about 12 years ago so my memory is lacking, but I don't recall it being a problem.

Charley
C R Huff

Okay, so far I've gotten the 2 brakes lines disconnected, the dust cover removed, the bolts removed from the forward facing bracket, and all but 1 bolt removed from the backward facing bracket.

Once the 2 brackets/supports the master cyclinder(s) are loose from the vehicle I anticipate that I try to get the bolts off of the brake master cyclinder.

But in looking at the brake and clutch master cylinders, I'm wondering if I have to remove the clutch master cylinder at the same time??????

I'm also thinking that I must remove the 2 supports as it won't be possible to get the brake master cyclinder out without either dropping the brake pedal lower so the push rod of the cyclinder clears the pedal. It doesn't appear one can simply raise the front end of the master cylinder up to get cylinder to clear.

1. Must clutch master cyclinder be removed at same time to get brake cyclinder out? It appears that if I need to remove the 2 supports/brackets, at the least the clutch master cyclinder needs to be disconnected.

My book simply says to disconnect lines and remove brake master cyclinder, not too detailed.

Thanks for assistance. Good thing its too cold to drive!
R.W Anderson

RW,

Might suggest that you take the cylinder off your car and take it apart before ordering anything from any body. If you have ordered the kit and have it next to you, but can return it if not used for only the postage in both directions, great! If you handy with tools, as most MG Enthusiast become, getting the master off the car is usually not too difficult. But too much time and effort spent getting them apart and back together successfully, and hopefully not failing (again), to make it reasonable although the cost of the rebuild kit is enticing. I believe getting these US spec dual master cylinders apart to be the worse.

As to the disassembly of the master cylinder, as John already pointed out, there is a chrome sleeve that is attached to the inside part of the cylinder, that cannot be damaged because it is a seal surface. If you are getting fluid down the pedal, it may be the seal that rides on this chrome surface that failed. However, you almost always have to maneuver 360 degrees around this chrome sleeve to break the plastic bearing and removed the lower “C” ring from the cylinder. All without scratching the chrome sleeve. Both the plastic bearing and “C” ring are down inside the upper bore about 1 inch. While you are getting out the bearing and “C” right, the chrome sleeve is constantly pushing up with spring pressure. I do not recall having seen any MGB masters that needed to be sleeved but the there is always a first. Also, the outer/larger bore has no moving parts associated with it. After you get the bearing and “C” ring out without scratching the chrome sleeve the fun is still not over. There is another spring on the part that will come out of the cylinder that has to be collapsed in order to remove a roll pin connecting the front and rear half of the piston. If the cotter pin like connection between the front and back portion is broken, I believe you can throw it all away as this piece is not available separately.

Some additional suggestions before getting to my point for providing additional, hopefully helpful info to you. You might consider loosening, not removing, the bolts that hold the pedal box to the firewall to aid in your removal and replacement of the master cylinder as well. Also suggest you keep the bolts that hold the master cylinder to the pedal box, loose or leave them out, until you get the brake lines started. You will need to removed the front push rod from the pedal are but removing the clevis pin and can push in the rod on the master cylinder in order to get it clear of the housing.

If you have been successful in getting it all apart, cleaned and inspected and ready to put it back together, open the box with the new kit and put it back together. A much easier road!

HTH

Fred
Fred Wright

Okay, after talking to someone with a 69B I confirmed I had to remove the entire brake & clutch master cylinders on my 72B while still attached to the metal brackets/frame, and of course drop the clutch and brake pedals from the assembly first.

I also learned from this same person that they wound up having to buy a used brake master cylinder to rebuild after they scratched the chrome piece inside the cylinder of their original.

So I've decided to take mine in to someone in TwinCities, MN or ship it to Apple to be rebuild.

Since I'm firm believer it replacing other things that are out at the same time (i.e. replacing clutch, then always replace throw out bearing at same time, that sort of thing); I'm thinking that with all the work it took to get the 2 master cylinders out I should have the clutch master cylinder rebuilt at the same time, whether it needs it or not.

Your thoughts?
R.W Anderson

I would rebuild the clutch master cylinder unless you know it was recently replaced or rebuilt.

Clifton
Clifton Gordon

And if you do that replace the clutch slave and flex hose.

Or is that shipwrights disease?
Paul Hunt2

Try this website for MG parts.

http://www.rockauto.com/catalog/raframecatalog.php?carcode=1290035&parttype=4152&a=FRc1290035k37837

There prices are very good. For example the 1972 MGB brake master cylinder sells for $154.00. As you probably know, most parts that are new are usually made by a single manufacturer but distributed by many companies. Master cylinders are a good example. Therefore shop price, not perceived quality.
Frank Grimaldi

This thread was discussed between 21/04/2008 and 04/05/2008

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