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MG MGB Technical - rear brake drums tight

Hello.
Please tell me if one should have new brake drums turned prior to installation. How much?
I installed new shoes and drums last night, but with MUCH effort. Adjuster nuts loosened all the way, both emergency brake cables disconnected, rebuilt calipers, all springs and tensioners installed properly, both shoes seated in caliper and adjuster notches correctly, and front and rear brake systems bled.
Car will not roll freely, smell of hot drums after returning from short trip to center shoes going forward and reverse directions.
Thank you for your advice.
Frustrated in Florida,
Craig
cwcolby

Drums should be turned if they are scored. It sounds like you might have the wrong brakes.
Bruce

Well, in The What It's Worth Department, both boxes the drums came in clearly stated "MGB" on them.
Help?
cwcolby

Craig. Back in the days when you had "factory" parts available--parts made by someone else for the factory and used on both the cars being produced on the assembly line and shipped to the servicing dealers as replacement parts, there was a great deal of control over the quality of parts and problems were quickly identified.

Today, parts may be made by a number of different sources and there is no MG factory organization which oversees the production of parts for "official/authorized" use. Thus, by mixing brands, it is possible that you can get a mis-match. With factory parts, you should never have to turn a new brake drum or radius grind a new set of brake shoes. With "aftermarket" parts, it is possible that you may have to.

So what to do? First, slow down and relax. Take a deep breath. This type of thing is frustrating, but it is something which can be handled. First, are you sure the adjusters were actually backed off fully. Some people replace them when doing a break job. I disassemble them, bead blast the aluminum portion, clean the steel parts, run a tap through the aluminum section, inspect all the parts, lub them and reassemble them. If you did not clean and lube the adjusters, they might not be fully retracted and this can hold the new shoes out too far.

Make sure there is a hair of slack in the handbrake cable. These things get adjusted to take up the slop in worn components and may have to be loosened significantly when a brake job is done.

Normally, if the brake drums will fit over the shoes, they are not too tight. You will have the adequate, if minimal, running clearance that is required for proper operation of the brakes if you can get the rear brake drums on without forcing them. If cannot get the drums on, it may require, if all else checks out good, that you have the new rear shoes radius ground to fit the drums exactly.

Speaking of fitting the drums exactly, most of us find that the shoes and drums are not an exact fit and require some "bedding in" until the external shape of the shoes matches the internal shape of the drums. Thus, for a while, the shoes are not making full contact with the inside of the drum and you will have slightly greater pedal pressure required on stopping. You might also generate a slight amount more heat because there are smaller areas doing the work. One way to check for this is to drive the car on a few short trips, pull the brake drums and look for the shiny areas. These are the areas making contact with the drums. Sand them down sufficiently to remove the shiny stuff (called glaze) and it will help bed the brakes in more quickly. Most of us simply bed them in while driving.

Bottom line, Craig. If the system fit together, you probably do not have an excessively tight set up. Drive the car for several short trips, over a couple of days, and see if the brakes bed in and become looser. If not, you can have the shoes radius ground to match the internal radius of the new drums (always modify the less expensive part, especially when it is a part that will wear more quickly over time). I suspect you did everything corrrectly and the edges of the shoes need to wear away a little to give you a properly set of bedded in brakes. If, after a couple of trips, everything still seems tight, pull the drums and inspect to see what may be dragging. Les
Les Bengtson

Any good brake or general auto machine shop should be able to "arch" the shoes.

This operation used to be standard procedure in earlier days, but is in less use as brake shoe manufacturing have become more precise, and brake shoe relining have become less common.
When was the last time anybody wanted your old brake shoes and pads as cores?

Bring both drums and shoes to the shop. The shop will measure the inside diameter of each drum,
then mount each shoe on a special swivel sander and shave the shoe a bit.

This will eliminate any high spots on the friction material and the shoes will be a perfectly matched
to each drum. Because of this, do not mix up the shoes and drums after they've been arched.

This should also relieve the brake tightness that you may be experiencing.
Daniel Wong

I went through all that you have described. Eventually thanks to some sound advice from the local MG guru, I replaced the single flexible brake hose located between the chassis and the rear diff and all works well. This hose looked fine from the outside and let the fluid through nicedly to set the brakes, but acted as a partial one way valve ie did not let the fluid back. This caused the brakes to drag and overheat. Change the hose first before you go to all the other possibilities.
Steve Dellaca

Thank you to all who replied so far.

Les, brake adjusters are bottomed out, i.e. the adjuster nut is so far unscrewed that both ends of the adjuster pistons are touching bottoms. Both ends of emergency brake cable disconnected from lever. If I smell hot metal from just driving around the block last night to try to seat and center the shoes, won't I warp the drums by driving too much? Breathing deeply here.

Daniel, surprisingly enough, the auto parts store from which I purchased the new shoes added a $5 core charge.

Steve, I didn't renew the rear brake hose, as both pistons in each cylinder are fully depressed, but your suggestion makes sense. I had the very same problem on my other B a few years ago on the front caliper, and most recently on the C clutch hose.

Unless someone else makes a different post, looks like next weekend I will have to pry off (pound off, more likely) the drums and see about that radius grinding or arching solution you suggested.

Les, please get back to me on that warping issue.

Any other comments or suggestions appreciated. Thank you. Craig
cwcolby

Craig. It is certainly possible to cause problems by overheating the brake drums. However, it is rare. How hot did the drums actually get and is it on one side or both sides? Are you sure the odor you are smelling is not paint or preservative burning off the drums? Certain high temp paints will release a solvent as they cure after they get hot. Certainly one quick way to check what is happening is to put the rear end up on jack stands and run the car for a short while, then see how hot the drums are getting. Les
Les Bengtson

Craig, I don't think it's a good idea to drive the car to bed the shoes. The drums should install easily with shoes centered and adjusters backed off. If not not there is something wrong, probably the replacement shoes even though it says MG on the box. Try Daniel's suggestion, if you cannot find a shop that can arch the shoes I would get some brake shoes from another source. The reason for arching brake shoes is that most brake shoes are standard size and if the drum is turned the shoes should be arched to match the turned drum. Brake drums do not have to be turned when shoes are replaced, however many repair shops will not warranty their brake work unless the drums are turned. Good luck, Clifton
Clifton Gordon

Take the shoes off, lay them inside the drums, and measure the inside distance between them. It will also show how round they are. Then file the contact points of the shoes (e.g. where the plates rest in the adjusters and cylinder) so that they will fit.

Tatty ( Non-member since 2003 )
Tatty

Les, smell no doubt came from the coat of paint I applied to the drums, I didn't drive so much as to warp the drums though.

Clifton, looks like all roads are leading to grinding the shoes.

Tatty, will check that suggestion this weekend.

Question for Bruce, Les, Steve, Daniel, Clifton and Tatty: how stupid would it be for me to grind off each metal end of each shoe a little, but keeping the original angle of the metal there, of course? That would cost less than taking the whole shebang to a machine shop. Or would that change the radius of the whole shoe?

BTW, $10.00 to arch the shoes at local brake shop.

Thanks, Craig
cwcolby

Craig. My experience is, if the drum will fit over the shoes, there is sufficient clearance and it will quickly wear in. While I do admit that others may have differing views, this system has worked well for me for many years. I might ask you to consider the fact that the springs retract the shoes to the limits of the rear wheel cylinders. Unlike the front calipers which do not have spring retraction, the springs in the system cause the brake shoes to fully retract when pressure is released. Now, when we use the brake shoe adjusters, we move only the top segment of the shoes outwards. The bottom of the shoe is not affected since it is connected to the wheel cylinder. Thus, as wear takes place, the bottoms move more than the tops and there is not equal wear across the entire surface of the brake shoe. When we adjust the shoes to take up wear, we move the upper edge further outwards which changes the previous relationship and wear pattern. Thus, the brakes are constantly "bedding in" as various relationships change. This is why it is wise to adjust the brakes on some form of regular schedule, as part of your periodic maintenance, is a good idea. It will maintain a more consistant relationship for a longer period of time. I check and adjust the rear brakes when doing an oil change/chassis lube every 3K miles. Les
Les Bengtson

This thread was discussed between 23/03/2003 and 24/03/2003

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