Been trying to sort this out for a while. Rear disc gets hot after a short journey. Master cylinder recently overhauled,new pistons and seals in caliper. When caliper is taken off disc only one piston moves when brake pedal is operated. If you then push it back in and hold it with thumb pressure and operate the pedal the other piston moves easily,but theres not enough action to move both pistons and also retract them both.Brake lines look quite old,and i am wondering if that could be the problem. Any ideas please? Cheers.
Sounds strange.
When you spin the rear wheel does it turn freely or is there a scraping sound?
Is the disc nice and straight?
Does it sit central in the caliper when you offer it up? Could the spacing of the rear wheel have gone amiss somehow?
If those both look OK, try splitting the caliper again and make sure the hole for the fluid is nice and clear, then rebleed the system in case there’s some air lurking in the bad side of the caliper.
Took on board what you said and started looking at the disc instead of the caliper. Fitted an old pair of worn brake shoes and rotated the wheel.l could wiggle the pad for most of the rotation then it went tight. The other pad was loose at this point but after roughly 180 degrees that went tight . Now got to find out wether it’s the disc or the wheel out of true.Have spun the wheel with a pencil against the rim ,not 100% l know,and it seems okay . Did the same with the disc and confirmed a
"High spot "in the rotation. Looks like a new disc is needed . Thanks for steering me the right way,
just got the problem of sourcing a reasonably priced disc now. Cheers.
Have you had the disc off lately, could it be a bit of muck behind the disc?
Or maybe you could find someone to give it a light skim to true it up. A lot of places don’t like doing cast disc’s as it buggers up there tool, but worth a try.
Have had the disc off when new tyres were fitted . Cleaned everything up before re-fitting ,but the problem was there before then…Not a lot of meat left on the disc ,so think l’ll look for a 2nd hand disc . Cheers.
I found that my SP front disc had different run-out in different positions. I measured all 6 positions with a clock gauge and set it in the best. The wheel was as bad as the disc.
Just realized l put brake shoes in a previous text. Must have been having one of those out of brain experiences.
A warped disc won’t cause excessive rubbing and heat generation, it will cause brake ‘knock off’ whereby you’ll find a whole lot more slack in the pedal (or lever if at the front) than there should be, even leading to requiring you to pump the brakes to get them to work. My rear brake is always warm after a run, I will have just used it to stop. Are we sure there is a problem here?
‘Not a lot of meat left on the disc’ – really? Mine’s done over 60k miles and I reckon is still thick enough to do that again.
I have recently got into the habit of checking all the discs after a run and the rear is very hot while the nearside front is cool and as they are linked l would expect them to be somewhere near the same. There is definitely a resistance between the disc and pad on each wheel rotation,it can be heard,felt and obviously generates heat and l am wondering how much would need to be skimmed to overcome this? Cheers.
Sorry – I was sounding somewhat over aggressive with my response (so unlike me). I’m just not convinced we’re not chasing a non-problem (triple negative – much more like me). Being bolted on to the drive box which will itself get quite warm will add to heat build-up here. If you can push the bike around on the drive OK, if you can turn the wheel when the bike is up on the main stand then I’d just be keeping a watching brief on it. I guess if it was always cold and it is now generally being warmer then something has changed though.
Thanks for your help. The bike still rides okay so no crazy panic . Hopefully it will get to the clocks with no problem and you can all have a spin of the wheel and see what you think. Cheers to you all
Worse than that, we didn’t notice. 
Reading that again – tight in one spot. That would rub of course if the piston is right back and the pad is against the caliper side directly. But sounds unlikely with worn pads. From new the callipers were shimmed to presumably centralised on the disc, so warped or badly mounted disc + no/bad shimming could be a cause? The only shims I’ve ever seen were real thin and I figured unworthy, so I always dumped them. If it is bad disc run out it would have to be massively out – see my earlier note on ‘knock off’, you’d really feel that.
Reading this again…
Simple test - on main stand, can you turn rear wheel by hand.
If yes, then it’s not binding. Common reason for disc getting hot is it’s binding, but in that case you would not be able to turn the wheel by hand, or at least not easily.
Maybe the problem is simply that the rear brake is doing more work than the front, so actually it’s the front caliper that needs looking at to see why it isn’t braking as hard. (?) (Moreover, it ought to be somewhat harder.)
By the sounds of your findings re pushing the pistons in and out, does suggest that maybe all the calipers could do with being dismantled cleaned up plus overhaul kits. Would also be a good opportunuity to replace the old looking hoses too.
Did I ever tell you the story (stop me if you’ve heard it)
1978, Triumph T140V, coming home from work, arrive home, go to wheel it backwards into its parking space, won’t move. Absolutely solid. Rear brake is stuck on, and extremely hot disc.
One of the roads I’d used had just been “resurfaced”, which even in those days amounted to little more than about 4 inches of loose chippings on top of wet tar. As now, leave it to the car tyres to “roller” it in. A tarry chipping had jumped up and glued itself between the brake pedal and frame at some moment when I had been depressing it. Literally one chance in a million.
Read an old road test of a V7 Sport in a magazine - the rider got a stone chip wedged behind the clutch arm at the back of the gearbox and couldn’t disengage drive.
Hi Red Ray,
Just caught your post and I think I may have come across this problem before. My Dad adjusted all the ‘slack’ out of the rear brake in order to get the height right. This had the effect of pushing the master-cylinder piston past the port where the fluid enters. The brake was free on setting off, but started to bind after a short journey.
I suspect the fluid expanded with no way to escape, pushing the caliper pistons against the pads. I think the factory setting is about 1mm clearance between the rocker arm and the piston. It’s also important that the pedal moves freely and returns properly.
I hope you get the problem sorted soon.
All the best,
Dave
All okay at the master cylinder ,it’s had a recent overhaul and there is the
1mm gap as required . Going to try the disc in different positions over the weekend, but think it’s too warped to make much difference . Been looking around for a second hand disc but they are rarer than hairy fish. Cheers
You could get the disc skimmed if it is not too buckled/thin?
Think it’s too warped .Took the caliper off ,put a bolt back in the hole and wound it in to touch the high spot ,rotated the wheel 180 and was able to get another 1/2 a turn on the bolt ,so don’t think there’s enough there to skim. Going to try shimming the low side with some very thin washers l have to see if l can cure it that way . Cheers.
Half a turn is about 0.5mm? Skimming may still be an option?