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MG MGF Technical - binding rear brakes

Just replaced both rear disc and pads. A good hour of the time was spent cleaning up the calipers and carriers, including removing, cleaning, greasing and refitting the guide pins. The contact surfaces of the new pads were liberally coated in copper grease and slid freely on the carrier faces. With the wife's help the pistons were exercised a few times, her pumping them out and me winding them back in. But having pumped the brake peddle to set the pads I found the pads were binding and some effort was required to turn either disc. I decided to refit the wheels and take her for a short drive (no more than 1/4 mile)to dynamically bed the brakes in. When I got back the discs were so hot I could barely touch them. With the discs cooled down and the car jacked up it still requires a bit of effort to turn the wheels. As a final task I had intended bleeding the brakes but even after wire brushing the bleed nipples and soaking them in WD40 I have only succeeded in rounding off one of the bleed screws so that hasn't been done, should it be? Does anyone have any thoughts as to why the pads are binding?
s Richardson

Almost certainly caused by the handbrake mechanism, which generally is in more need of exercise than the piston. Now that teh piston is wound back in to 'new pad' position, the handbrake mechanism is back in a position it hasn't been in since new pads were last fitted, and in the ensuing period corrosion does its thing meaning the handbrake will go on OK, but there's too much resistance from the corrosion for it to release fully.

Best done with the caliper on a bench so you're able to wind the piston back in repeatedly to speed up the exercise, but exercising the handbrake mechaism on the car can be successful. Remove the return spring & work the lever to and fro while squirting lubricant into the area around where the shaft goes into the casting, and if you're lucky it'll gradually become less and less resistive, and if you can get to the point where you feel a 'click' as the lever returns to the 'rest' position then you're done, the binding will cease.
bandit

Thanks for that Bandit
It makes sense so that will be today's exercise. However things have moved on some and your thoughts would be appreciated. I said I checked the wheel rotation after going for a run, well while I had the jack out I jacked the front up to do a comparison. The OSF rotated freely but when I checked the NS wheel it had as much if not more rotation resistance than the rear ones!!

Although I have never experienced any pulling to the left I suspect the wheel has run really hot at some time as the caliper, which was as corroded as you would expect having spent 11 yrs on there had an area free from corrosion and light grey in colour. If you have ever applied a blow torch to old rusted iron you will know what I mean.

I carried out the same removing, cleaning, exercising and refitting as for the rear wheels. The disc grated a bit when I rotated it but like the rear ones when I tried to rotate the disc after pumping the foot brake it was locked solid. I've found that judiciously tapping around the caliper and carrier pin housing frees the disc until the brake is pumped again.

My thoughts now are, although I can push the pistons back mechanically can the hydraulic system manage to do it? Is it possible that the brake fluid has boiled and I've got gas in the system

s Richardson

There is just a single link pipe from front to rear, so if there was a gas content issue that affected both front & rear it would be having the same effect on both sides i.e. all 4 wheels. And I'd expect there to be a noticeable feel in the brake pedal i.e. almost at the floor before hard braking can be achieved. The symptoms of the front caliper are typical of a corroded piston surface, lifting the dust cover off and applying lubrication is likely to sort that, or getting the piston well exposed so rust can be sanded off, and I'm still fairly sure exercise of the rear handbrake mechanism will resolve the problem at that end.

The rust-free spot on the front caliper could also be where someone has started stripping the rust off so the caliper can be painted red, a common 'mod' that has arisen out of the big red AP Racing calipers fitted to the 160 versions. I suspect any binding that has got the caliper to blowtorch level temperatures would have manifested itself already, huge clouds of steam at every puddle for starters... ;0)
bandit

Cheers mate, I'm already working on the handbrake linkages and as soon as I've finished there I'll return to the front and do as you suggest.
s Richardson

Hi Bandit
Taking your suggestions in reverse order:-
1. Front caliper, I peeled the dust cover back and you'd guessed right there was slight corrosion on the piston surface. A clean with some crocus paper and a bit of lub has sorted it.
2. Hand brake, working with everything still attached to the car I removed the spring on the NS brake and used a spanner on the hexagonal bolt head to work the lever back and forth. The lever moved freely as I rotated the spanner anticlockwise i.e. into the car but when I brought it back through its start position to work it in the opposite direction there was a solid resistance. I assumed this was the lever hitting up against the internal operating mechanism. I know it would be difficult to do but not wanting to cause any internal damage I didn't force it and just worked the lever back and forth between these two positions. I found the same condition when working on the OS brake. After pulling the hand brake on/off a few times I found the discs turned easier than when I first fitted them but I could still hear a light scrapping against the pads. Wheels back on and after a short run the disc were still hot to the touch (Could hold my finger on them for about 3 secs compared to 6-8 secs when touching the front ones).

At no time did I hear the reset click you mentioned.
2 questions: Was the restriction just the internal workings or should I have forced against it? and should I have done it with the pads removed so I could wind back the piston as you mentioned in your post?
s Richardson

It's ideal to do on a bench, as then you can set the angle to help lubricant trickle into the caliper body around the handbrake lever shaft, but if your ability to re-bleed the system afterwards is currently up aginst a rounding bleed nipple, I understand your reluctance... Were you using a specific brake spanner, i.e. one with more flats and slightly wider profile? They're worth every penny, and can usually undo nipples that have rounded with a conventional open-ended spanner.

The point where the lever shaft goes into the caliper body is concealed under a cam assembly, this is pressed onto the shaft so will come off (generally requiring an awful lot of persuasion, and swearing helps too) and will reveal a direct path you can work lubricant down, and success will arrive much quicker.

It's the ease of the lever movement away from the car that is critical, that's the direction the lever moves in as the handbrake lever is released, and that's where you're looking for the click as it reaches the 'rest' position. It's a pretty robust mechanism so the chances of you snapping anything are slim, exercising it with free movement of the piston does help so taking the disc & pads out of the way would be worth doing. And do the same check for corrosion on the piston surface too, on a completely un-corroded i.e. now caliper the piston is very easy to spin back in, you can almost do it with fingertips, so if using the windback tool still requires a fair bit of effort then that's good evidence that friction exists between the piston and its surroundings. And the amount of friction that can leave the pad dragging on the disc face is really quite small...
bandit

Bandit you a wealth of valuable information , Thank you.

I think i will not try and rebuild my rear brake calipers, ( they failed the MOT today because the efficiency is low.. only 15 percent.. :( ) and that is what the garage say is the cause... duff caliper

though i think i could take the assemblies apart, and fit new parts.. making sure everything is A1, may end up me spending as much money and valuable time as buying a ready to fit item...so i am looking at what Rimmer offer, and just knuckle under and pay the price..
N.J. Simon

No.. all you need is enough to hold the pads asganit the calipers piston nottoo much glue when next time you change them they should come off easy.@dammitcrapLubricating the bolts is very important because you want the bolts? to slidback and forth easily not to be dry .and yes the caliper bolts.Any high temperature caliper lube will do just fine.Are you an installer?
James Neel

This is an extremely poor exlmpae of doing a brake reline! If all you care about is cost, then go ahead and follow this garbage. If you care about doing it properly, being able to spot potential problems, and want your new pads to last more than a month, get a $25 repair manual from an auto parts store and read it first. There is so much more to doing a proper brake reline than this video shows.
James Neel

This thread was discussed between 25/03/2012 and 22/07/2012

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