Understanding MG Brakes

In this article I would like to deal with the most important part of your MG - or any other car for that matter. No!, how clean your car is not the most important thing. Neither is how well the motor goes.

The most important part is the brakes.

Without these working properly, you won't be going very far. Usually it is the rule that people do not get any work done on this most important piece of the machinery until something goes wrong. The need for maintenance is usually long before faults become apparent. The most common excuse used in making an Insurance claim is THE BRAKES FAILED. This usually means that the person driving didn't know how to control the car in an emergency stop situation.

It is generally accepted that hydraulic systems should have the seals replaced every 25,000 miles (40,000 km) or every two years. I know this doesn't seem like very long but you'd be surprised how often and for how long faults can go undetected. The best way to prolong the life of your hydraulic systems - brake and clutch - is to siphon the master cylinders empty and refill with new fluid and then pump clean fluid through all the other cylinders. Having clean fluid in the system ensures that contaminates such as water and other foreign matter that works it way in past the seals is removed. Brake fluid is very hygroscopic and will draw moisture in through the brake hoses. Fitting Teflon braided hoses eliminates this problem but they are hard to inspect for deterioration such as hardening with old age and they don't last forever.

I generally make Easter and Labour weekends the approximate time to undertake inspection and maintenance.

So to start at the beginning for your maintenance check:

Remove the master cylinder cover and pull back the boot on the pushrod and check for leaks or rust. If there is rust or a lot of gunge in the outer end then it is time for an overhaul. The circlip and piston can be removed and a new set of seals fitted without removing the cylinder if the cylinder has no rust evident. If there is any rust then the cylinder will need to be honed and checked for pitting. This can be done in place but it is better to remove the brake cylinder for better servicing. Sometimes this pitting is only at the very outer end and not where any seals work. If it is further in than 3/8" (10mm) then throw away the cylinder or send it out for resleeving in stainless. 'Just Brakes Ltd' do a good job. Any pitting on the bottom of the bore can be disastrous in a sudden braking situation. it could be that maximum pressure comes with the cup right on the pit mark and then it is possible that it would blow a hole in the cup.

The MGB up to 1976 had a separate booster mounted on the LH side of the firewall. The seals in the booster obviously have the same life span as the rest of the braking system. The booster is easily dismantled but unfortunately the seal kits are very expensive. The inline booster on post '76 models is easily removed for servicing without having to upset the diaphragm area. After new seals are fitted and the cylinder is back in place, bleeding can be difficult as they tend to suck air back in through the 'brake failure' switch.

Rear brakes - I reckon that about 75% of Bs that come in to our shop have had the rear brake incorrectly assembled. The lower spring has the long hook fitted through the slot and not the round hole. The easiest way to remember how to fit the shoes is to imagine that the forward movement of the car has dragged the lining to the end of the shoe; ie: on the RH brake, the rear shoe has the lining at the top and the front shoe has the lining at the bottom. The springs are fitted behind the shoes. This is known as the twin leading shoe set up. If they are not fitted correctly, the efficiency of the brake is reduced and if one side is different to the other, then a crash stop on a wet road will almost always result in a spin. Not a happy situation. If the brakes lock up under heavy braking, get your foot OFF the pedal and apply the brakes again. This very hard to do if you are heading for a sudden stop against something that you'd rather not be stopping against.

If the brake drums are scored or just badly worn, then they should be machined. My experience has shown that on most drums the centre hole does NOT run true with the locating spigot onto the rear hub. This means that all the brake machiners cannot get them to run true. An out-of-true drum will cause a brake to grab under heavy braking. I have set up my lathe to locate on the spigot and not the centre hole. If the brake cylinders are leaking then usually it is a waste of your time cleaning them out and trying to find new seals. In most cases they are rusty. If you are replacing the cylinders on a '72-'74 chrome bumper MGB that has 7/8"cylinders then it is a good idea to replace these with .8" cylinders. There was a service note that came out in the early '70s advising this to be done as there was a tendency for the rear to lock-up under heavy braking - especially on wet roads. This was because of a change in front brake pad material to a harder type of lining.

The front brake callipers are pretty fool-proof but never-the-less do require attention from time to time. All our MGBs and Midgets are getting old now and some have never had the callipers looked at. Usually the seals will need replacing every 2nd set of pads as dirt does get behind the pistons and make it hard to push them back. A good test of the calliper condition is to jack up the car, kick the wheel a couple of times and see if the wheel spins freely. If it doesn't then the seals are hard and distorted and therefore they don't assist the return of the piston. The bottom of the seal groove has a step in it which distorts the seal when the piston is pushed out. If the wheel bearings are too tight and haven't been set up with .002" clearance then the pads will also tend to drag on the discs. This not only causes excessive pad wear but also costs fuel and we all know that that is too expensive to waste.

The last part of the braking system to be dealt with is the PARKING brake. Yes, the hand brake. This is also an emergency brake. If you have to use it for emergencies then you are sadly neglecting your car. Don't abuse the cable by hauling the brake on every time you park the car even on the flat. Just leave it in gear. The cable should have been disconnected during the repair of the rear brakes to ensure that the cable is free to move and also so it does not interfere with the brake adjustment. The brake adjusting screw should have been wound right out and the thread cleaned. If the adjuster is hard to screw in during normal adjustment of the rear brakes when no other repairs have been done, then undo the screw and clean the thread. Dirt accumulates on the thread and then it seizes and alloy from the adjuster body sticks on the screw. This can be picked off using a sharpened concrete nail or if you are lucky enough to have one - a thread file. The adjuster is easily removed without dismantling the whole brake although I have seen the nuts rusted on and then break off the stud. Soak them in penetrating oil first. This problem can be reduced by putting a piece of plastic tube on it after adjusting. Later models had the length of the thread reduced to help. A stripped adjuster can result in the sudden loss of brakes in an emergency. After adjusting the rear brakes, reconnect the cable. If the cable has run out of adjustment but is still in good order, then the anchor point can be moved back on the tunnel. Simply slide the seat fully forward and undo the nut, drill a new hole about I" further back and refit the anchor point. Don't leave off the spring and washers on the adjuster, (the large washer goes against the lever) these help the cable to return. If the cable has not been greased often enough then it is very difficult to get grease back through it and almost impossible to clean out the dirt properly - even in a big cleaning tank. Most of the new cables have a plastic sleeve over them and this prevents dirt and water getting in through the casing of the cable. New cables really are quite cheap. A stiff cable wears out the rear brakes quicker and also costs fuel.

Garth Bagnall

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