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MG MGB Technical - brake cylinders

i have leaky rear brake cylinder. is it ok to just replace one, or is it recommended both be replaced? they are about 7 years old.
1977 mgb
randy olson

Let me be a little more articulate now that I have a regular computer keyboard and not the laptop.
I have one rear brake cylinder that's leaking. I already removed it.
I ordered two, one for each side, because the sales person said it was better to do them both at the same time. He reasoned that the other would probably fail soon, so you might as well do both while you are bleeding the brakes.
However, the driver's side cylinder still looks good. And, it's kind of a hassle to work on these drum brakes.
I have yet to replace the springs that hold the shoes on to the backer plate, but it doesn't look like fun. For one thing, the springs are BEHIND the shoes, making it hard to get the brake pliars in there...any tricks?
So, I could replace both cylinders, but if I don't need to I'm thinking about saving one for a spare.
Another issue is the brake shoes that were on the bad right rear. The brake surface got a lot of brake fluid on it, so I cleaned the shoes and drum well with brake cleaner. They don't look too bad, but the sales guy said if they were wet they should be replaced. They have a lot of thickness to go before they are worn out.
So, it's down to a question of replacing the one cylinder, or replacing both, plus possibly replacing the wet shoes, then you have to do both shoes, then you need a new spring kit, get the picture.
Let me know what you think I should do, and if there are any tricks or good sources for putting the brakes back together.
1977 MGB
Tahiti Blue/Tan
Twin SUs
randy olson

So, I bought two brake cylinders, and two sets of shoes. The spring sets were obsolete, so hopefully the old ones will work.
Does anyone have any suggestions for putting the new brake shoes on? I'm concerned because the springs that attach the shoes are on the back side of them...hard to get a tool in there between the backing plate and the shoe.
Any suggestions or words of encouragement are appreciated.
randy olson

Replace both cylinders as advised by your parts sales person.

It's been 7 years and you don't want the other cylinder to leak
all over your fresh shoes and ruin them as you will then have to
replace all the shoes (on both sides) to maintain balance.

To assemble the brakes themselves - you install one shoe at a time.

Attach the upper and lower return springs to one shoe and install it
onto the backing plate (with retaining pin, spring & washer). I always
choose the shoe that accepts the lower spring with the long-legged end.

Hook the other ends of the upper and lower return springs onto the
remaining shoe while it is still not installed (free floating).

Stretch the remaining shoe into place on the backing plate.

No special tools needed here. Just use your muscles.


If the old wheel cylinders don't look too bad,
they can be rebuilt using the appropriate kit(s).

Daniel Wong

i had one leaking but took advice and replaced both wheel cylinders. and hopefully, because of eclips, will not have to do this again for sometime. i recommend getting one of the e-clip tools or getting a set of instructions on how to adapt a socket etc for the job (used this, very helpful, cant find copy now though), as i found those clips to be fiendishly awkward to replace. its one of the few jobs that drove me to triple swearing, indeed, i was forming whole sentences of expletives as the rain trickled down my nose, as the eclip leapt across my garden for the 10th time. then again, i make no bones about my inexperience of mechanical matters, so maybe i was making a big job out of nothing. the springs were no problem - one of the few times where the manuals description of rerassembly is dissasembly in reverse was useful.
m rae

Another bit of advice for new do-it-yourself brake installers is to do ONE brake at a time. Use the other brake to guide you if you get lost.

Those e-clips are a BEAR to install. I dread even thinking about it.

Reading in the MG owners club magazine from earlier this year (can't remember which month) There is a tool for putting those horrific clips on.
T Kendall

Hey guys,
I installed one of the new cylinders last night, and I understand what you mean about the E clips. Those are the fasteners for the cylinder, right? I pushed mine into the sludge and around the cylinder protruding through the back of the backing plate. While the scum held it there I used a small hammer to drive it on. Only lost it once when it bounced a few feet away.
Daniel Wong's suggestion of installing the springs first, then muscle-ing the whole brake shoe into place is intiguing. It doesn't sound feasible with just your hands. I wonder if a pry bar would be helpful. On the other hand, Mick says the springs were no problem. I'd like to know more on his technique. What book describes the installation best?
I have only one rear brake apart, and will do it first so the other can be used as a guide...good idea. I also took a few digital photos of the existing set up just in case.
1977 MGB
randy olson

I have the new cylinders and brake shoes on the MGB. Thanks for all the tips!
Next, I need to adjust the brakes. I can't get the drums back on.
I have a tool that I believe is for adjusting the brakes. It's a box end wrench with a square hole of different size in each end. However, neither end of the wrench seems to fit the adjuster bolt...either too big or too small. I can probably use an open end wrench.
Are there any tricks to adjusting the brakes?
Do you bleed the brakes before adjusting? Do you need to have the drums on to bleed the lines?
Thanks again,
1977 MGB
randy olson

Randy; You need to back off on the adjusters until the brake drums will fit, you may have to bump the shoe assembly up/down or left/right to align it so the drum will fit. After the drums are on you need to adjust the brakes, after they are adjusted you can bleed them.

Clifton Gordon

Thanks, Clifton.
There's not much in the manuals on how to adjust the brakes. Can you briefly describe the process?
randy olson

Randy. There is the brake adjuster. It is, simply, a beveled piece that moves outwards to take up the "slop" that developes as the linings on the brake shoes wear. When first setting up the brakes the adjuster should be set so that the adjustment piece is as far inwards as it can get. This allows you to easily slip the brake drum onto the shoes.

As the shoes wear, the adjuster needs to be moved outwards which, in turn, moves the brake shoes outwards to take up for the wear. The wrench you describe, a 1/4" square drive, is used for this purpose.

Install the drums. Turn the drums and adjust the adjustment mechanism until you feel a slight drag on the drums. Back the adjuster off by one "click"--you should be able to feel the clicks and a little anti-seize on the adjuster engagment surfaces helps. Install the wheels and go driving.

Brake drums are "turned" to make them concentric. If not, they might be a little out of round. This will be noticed by the adjusted brakes making contact at some points, then being looser at other points, as they are turned. Minor warpage of the brake drums is normal--it cannot be felt when braking. Major warpage will result in a pedal which pulses when you are braking. Minor warpage, a few thousanths of an inch, seems to be fairly normal. Major warpage is a problem that needs to be corrected.

Thus, when turning the brake drums by hand, while adjusting them, it is common to feel a tight spot when adjusting. The amount of acceptable warpage is only determined by experience and cannot be give over the internet. If you can lock up the brakes in one position, but they are free beyond that, it is time to have the drums turned. If they are somewhat tight in one position and loose at another, this seems to be acceptable.

When drums have been turned, unless taken to an "old man" who knows what he is doing, the edges of the shoes will wear more than the center parts. Thus, more frequent adjustment is required when the new shoes are first fitted until they wear into full contact with the drums. Something over 35 years ago now, I took the drums of my A-H Sprite into such an old man and he turned the drums, then matched the new shoes to the drums by radius grinding. They made full contact from the day they were installed. I have not been able to find such a specialist for many years now.

So, back off the adjusters, install the brake drums, adjust until there is a light drag, back off "one click" and check, and adjust, your rear brakes over the next several thousand miles. About every three months works for me.

Les Bengtson

well, this a first - someone wanting to know how i did something! not sure i should say - dont want bad habits spreading further than my garden! i am sure this isnt the 'correct way', but it worked for me (sometimes ignorance works i guess!). i replaced the new pads and springs etc by assembling them on the floor as per the side i hadnt touched, backing off the adjuster, then lifted the whole lot up, mounted on one side and with a little pulling got the other show into place in one go, then fiddled about till i got the hand brake mechanism in the right place. probably what Daniel describes above more eloquently than i am doing here. was only a couple of months back, but went so well that i cant remember any more detail, only that i tried it by putting on the shoes then springs and that didnt work for me, so the 'preassembly and chest expander' method was used. didnt take too much force (i am no body-builder!) anyway, right or wrong method the end result was they work good (and to sort my concerns over my incompetence i got them checked out by a mechanic mate before i drove the car - paranoia has a useful place where myself and mechanical things are concerned!).
glad to hear the world has tricky times with e-clips, someone has suggested replacing with a conventional cir-clip, but if nothing else, anything that is a tricky and tight to install as an eclip is presumably also a very strong hold. and my 3 year old has this job to thank for his ability to now swear alongside the best of them (thankfully wearing off).
m rae

OK, guys, I need some advice.
I replaced the rear brake cylinders and shoes. I put the brake drums back on and it was a very tight fit even after backing off the adjuster as much as possible. Then I tried to bleed the system, and that's where things are getting strange.
The front calipers were easy to bleed and seem normal. However, the two back cylinders don't bleed fluid, just air comes out the bleeder! I first tried a vacuum pump, but later tried the old-fashioned way with my wife pumping the brake pedal while I opened and closed the bleeder. Still didn't work. I pumped it for a long time thinking that maybe there was a lot of air in the system, but that didn't help.
Then I tightened the nut on the brake line to each cylinder. Didn't help.
From there I checked the entire brake line system from the rear wheels to the reservoir. The only suspect seemed to be the flexible rubber hose at the rear axle. There were no leaks apparent in the hose, but I thought I might have seen a crack or two...nothing that looked serious. I then disconnected the flexible hose from the metal pipe going to the reservoir, and checked to see if I could vacuum any fluid out of the pipe. No fluid came out when I pumped it with the MitiVac.
The brake pedal basically goes to the floor. I think the fact that the front calipers are holding pressure is the only thing keeping the pedal from going all the way to the floor.
So, as you can tell, something is not right. Any ideas what it is?
1977 MGB
randy olson

Randy; Your car has a pressure differential warning actuator in the brake master cylinder. Look in the Haynes MGB manual on page 149 section 7. "Bleeding the hydraulic system (Pressure Differential Warning Actuator in circuit)" Note in 7. 1. "but before commencing to bleed the system, take the leads from the pressure failure switch and unscrew it 3 1/2 turns to bring the plunger clear of bthe piston." The PDWA is item 2 in figure 9.13 on page 157. Be sure the cylinder reservior if filled with fluid. Try bleeding the rear wheels, they should bleed OK. Follow the bleeding instructions in sections 6 and 7.

I have seen two people spend almost a day trying to bleed rear brakes on a Triumph TR-8 when they didn't rremember to loosen the PDWA.

I hope this information helps.

Clifton Gordon

Thanks, Clifton.
Make that three people. My wife practically went to sleep at the wheel while she pumped the brakes!
Do you know anything about Jaguar brake systems?
randy olson

Randy, I have no Jaguar brake experience.
Clifton Gordon

When I tried to turn the pressure failure switch it broke off. So I removed the master cylinder and dug out the rest of the plastic thread portion.
Now I either need to buy another switch, repair the one that broke, or just plug the hole with a bolt. Is that an option? Or does the part that sticks into the master cylinder do something needed?
The previous owner had already disconnected the wiring from the switch, so that is not a problem.
I just want to bleed the brakes!
Let me know what you think.
randy olson

Randy; I don't have a rubber bumper car but if you use a short plug it should work, but you will not have a warning indication if the brakes should fail on one end of the car plus the actuator will not be available to stop fluid flow to the defective circuit. With the wires disconnected you didn't have the indicator but perhaps the actuator was working. If it was my car I would try a plug and see what happens, that's what I would do but I'm not suggesting you try it. If you do get the brakes to bleed make sure the brakes on all four wheels will lock, car on jack stands and try turning the wheels while your assistant is holding the brake pedal down. If you pass that test try a test drive in an area where there isn't lots of traffic until you are sure the brakes work OK. I'm not totally familiar with the fluid flow path in a master cylinder with a PDWA and I'm concerned there may be some feedback with the actuator removed that will cause the brakes to not work, that's why I'm suggesting the on stand checking before driving. If you don't feel comfortable doing something safety related I would advise getting some professional help. Sorry I can't be more helpful. perhaps someone has done what you are asking and will respond.

Clifton Gordon

I must say, this thread makes me appreciate even more the lovely hydraulic simplicity of the original, non-servo, non-acctuated setup of the early cars. Fewer parts to break (brake, ahem), and it's just you mano-a-mano in 'direct' contact with the pads and shoes via fluid dynamics. Poetry.

So...I'd personally vote that removal of all complex later braking 'aids' would be one of the best mods I can think of!
Curtis Walker

I bought a 3/8" fine thread bolt and will try plugging the hole where the sensor was.
If anyone has any experience or knowledge of what the pressure failure switch on the underside of the brake master cylinder does, besides sense low pressure, please let me know.
Also, if anyone has a spare sensor they don't need, please contact me off-list. If the bolt doesn't work, I may need another sensor.
randy olson

This thread was discussed between 18/04/2006 and 25/04/2006

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