I have recently overhauled my rear brakes, replaced both calipers, off side metal brake line and both rear rubber hoses are new. It passed the mot a month ago.
Today the rear brakes are well and truly seized, so bad infact that smoke was coming from them when I got home.
I pulled over and undid a bleed nipple to release the pressure, this instantly worked and the wheel turns freely, but not for long and the same thing happens.
I have had both wheels off witth the car standing on axle stands, when the handbrake is appied and released they stick, I can turn them, but its difficult, undo a bleed nipple, they turn freely. This exact same thing happens when you apply and release the foot brake.
When driving the car the brakes gradually stick on more and more (even with out applying the footbrake) until eventually you cant drive it and I have to pull over and release a bleed nipple to carry on.
Any ideas would be much appreiciated....

Quick diagnostic question first.

Are the rear pads wearing down evenly (inside pad vs outside pad)? It is probably a bit early to tell yet but if they are sticking that badly you should be able to see something already.

Be careful - if the pads get too hot they can ignite surrounding materials (like grass or excess brake fluid). If they've been hot enough to smoke the odds are the pads will need replacing anyway.

Best guess given what you've said so far is either the sliders are jamming solid, there is some kind of blockage in the brake line or your handbrake mechanism is faulty and the springs on the cable are continually pulling the brakes on. Remember you need to wind the rear pistons in to move them back once they've gone past the first mm

Try unscrewing the brake light switch slightly, seriously. Had this before after a brake overhaul and bleed. Switch may have been adjusted down to work with old setup, and now you have less travel, which restricts the return of the brake pedal.

Thanks Guys, I had kinda come to the conclusion that it was the master cylinder as its both rear calipers that are sticking on. The handbrake operates freely and the sliders are greased and free moving (I can pull them out by hand!)
Do you reckon this is a possibility?
I like the idea of adjusting the brake light switch, Oh how I hope its that!
That would save me a few hours hard labour lying on my back with my legs in the air trying to undo seized brake lines! :-(
Could the seals in the calipers be toast as well as the pads?
There was quite a lot of smoke!

Normally if the seals are gone then you don't get any pressure in the line but of course the master cylinder is two in-line cylinders in one so strange things really do happen.

Just taken one of the rear calipers off, there is no fluid leaking out of the rubber hose? Could the new hoses I fitted (well they were NOS) be faulty. No idea how old they were as they came with a job lot of stuff I bought off someone.
Plenty of fluid in the reservoir.

My personal preference has always been externally braided lines as they rarely go wrong but the MOT testers aren't fond of them, you have to make really sure they aren't rubbing against anything. On the plus side they don't disintegrate internally and don't balloon so the pressure is always consistent.

I'm still confused on this one though. In order for the rear callipers to keep locking up they must have some pressure in the line that isn't being released.

If the pedal was not returning properly it shouldn't lock things up, just keep the pads dragging ever so slightly rather than building up pressure and more importantly it would effect all four corners not just the rears. The fact that it is just the rear lines means it has to be the callipers, the lines (including the junction) or the master cylinder.

I would still try to eliminate the handbrake as the cause by physically disconnecting it from the callipers and seeing if it still does it (just remember to refit it afterwards and don't forget that you haven't got a hand brake and avoid stopping on any hills).

Well I have just replaced the rear calipers, I figure the seals are knackered now any way if they overheated so much that they smoked!
Now in the process of bleeding the brakes, the fluid is like soup! I mean it is thick, creamy, whiteish. Is this a result of the overheating?
No wonder it didn't drip out of the hoses! Its too thick.

I am now hopeing that I had a dodgy caliper, it siezed, overheated big time, turned the brake fluid to soup and this spread to the other caliper?

Is this as far fetched as it sounds?

I will definately road test the car with out adjusting the handbrake back up. Then road test it again with handbrake adjusted.

Sadly brake fluid doesn't normally move like that. Once it gets to the calliper there is very little back flow unless you have some recirculating brakes and I have never seen those outside of motorsport.

If the brake fluid is that thick on both sides then it could well just be contaminated in which case you need to do some serious flushing of the system.
You could well be right on the seals but they may surprise you.

What grade brake fluid did you use? What grade was in there before?

There was grade 3 in it oroginally and when I overhauled the brakes I used grade 4 synthetic.
I have road tested the car with the handbrake cable loose, it was fine.
Adjust hand brake and its sticking again, (not sure if this is conincidence as the fluid still appears to be sludgey) undid a bleed nipple and it instantly freed off.
But milky brake fluid is still coming out after using a whole container of dot4!
It seems the brake fluid is contaminated throughout. Is mixing these causing the problem or has water got in some how?
If its water, I can see that when it get hot it expands and causes the brake pistons to move out to the pads.
More bleeding me thinks...but how am I going to successfully flush the system?
I dont think bleeding it is going to work.

Dot 3 and 4 can be safely mixed together. If it was just water it would lower the boiling point and you would find the brakes won't pressurise - the complete opposite of what is happening. Visually there is no way to identify brake fluid contaminated with water.

Even if the fluid was just overheated it would be darker and the consistency should not have changed but *I could be wrong*

The big worry is that the system is contaminated with oil, if this is the case then you will have to replace your entire braking system as there is no practical way to clean the contaminants out, flushing won't work and the rubber components will fail over and over again.

What you need to do is get some of that fluid bled off so that it can be tested. If it is contaminated with oil there isn't much can be done but if it is something else it shouldn't be so hard to sort out.

Just a thought - when you got the callipers (or serviced them) were they painted?

I have to say I dont really agree with what your saying at all.
I have done a test and put water in brake fluid...it instantly goes white, milky in appearance. This is the symptom I am getting.
If there is water in the brake lines, water expands or gives off steam at any where near 100 degrees, if it gives off steam it can only expand and push the pistons onto the calipers.
How on earth would oil get into the brake fluid? Water, I can see, if I have a cracked brake reservoir, which I do.

I did some digging on the subject at the weekend and the rep from Lucas Oils (the best expert I could find) suggested water but it would have to be very heavy contamination.

At about 15% to 20% water content the brake fluid just looks like brake fluid... at higher levels of contamination it might be different, and your experiment probably proves the point.

The problem with steam in the pipes is it is under pressure so the water won't boil until well over 100degC and the steam is compressible so you find the pedal goes very soft however the gas expansion could be sufficient to move the pads a tiny amount and that is all it would take.

It is not unheard of for people to use transmission fluid in the brake system or even engine oil, the end result is irrevocable damage to the system that requires the whole system to be replaceed

Do you have the means to pressurise the brake system from the reservoir? I would start with that and trying to get clear fluid at the calipers. I once had a similar problem on a different car and it was the master cylinder by my fluid was not contaminated. Are the self adjusters working OK and are the pads properly located so the pin on the pad is in the slot on the piston (if you know what I mean)?