I'd think the spring-returned MC piston might draw some hydraulic fluid back into the master cylinder, creating a negative pressure which withdraws the caliiper piston a little.

Its called fluid displacement, but the amount of fluid needed to push the piston onto the pad and onto disc is very small and doubt its enough to cause negative pressure to withdraw the piston. and as in most cases its traveling uphill to the MC, I doubt that will work

but of course the fact that its on the Internyet doesn't mean its true.

As long as it works it doesn't have to be believed

If there is a return spring in the master cylinder

AFAIA there is a return spring as I have fitted many over the years, though most reseal kits do not come with one or never used to?

Its possible your correct?. but imo there is not enough movement there to cause calliper piston to move back, it would need something more substantial, ie rubber seal that acts like a return spring, which it does!

Don't forget that when the MC returns to its normal position, it uncovers a fluid route back to the brake fluid resovoir.

This is so that fluid makes up any volume from the brake pad wear taking place each time the brakes are used.

So there can't be any negative pressure there.

Good point, though it would be nice to hear how the OP got on in fixing the fault?

Don't forget that when the MC returns to its normal position, it uncovers a fluid route back to the brake fluid resovoir.

This is so that fluid makes up any volume from the brake pad wear taking place each time the brakes are used.

So there can't be any negative pressure there.

Yeh, thats a complication, but I'd think there could be a transient negative pressure while the piston returns to the rest position and uncovers the transfer port.

Can't see how there couldn't be, because (unless there's a non-return valve, which in my car I THINK there isn't) if it can't generate a negative pressure, it can't generate a positive pressure, and then it can't work.

Moving the (half-assed?) speculation a bit closer to the OP's concerns,

IF one of the pistons (or seals if it was a bit loose on the piston) hangs up in the MC bore and doesn't return fully, it might maintain positive pressure in that brake circuit.

IF the brake circuit split was front-back (I've always assumed diagonal was commoner, but I understand both exist) then that might stick your front or back brakes on.

IF you cracked the bleed nipple and your brakes freed-off, that'd suggest they were stuck on due to hydraulic pressure, though it wouldn't tell you why.

The reported fact that both brakes stick on suggests a hydraulic pressure cause for the problem.

The master cylinder seems to be a common element and potential single point of failure.

Edited by edlithgow on 13/12/2018 at 23:50

The reported fact that both brakes stick on suggests a hydraulic pressure cause for the problem.

Yes probably as Hardway mentioned being a possible and likely cause.

Hardway mentioned a "faulty/seized regulator valve maintaining pressure"

I'd guess that's the proportioning valve. Different. to a hung-up master cylinder piston.

Could, I'd think, be either.

Which is most likely? Dunno. I'd probably have to "overthink it" to do other than guess at that.

Proportioning valve is probably cheaper but I dunno what they're like to take apart.

sorry for my absence, brakes stuck on for the fourth time, its intermittent fault, could drive for2hours or 20 minutes before it occurs. it stuck on and made I t to main dealers, who are to busy to look at it now.. book it in for next day. go home and bleed of the brakes again, the fluid was previously in need of change. next day back to main dealer, try as hard as i can, brakes won't stick, so i go home again, later on they stick, return to dealer. they pressure bleed system, but can't find fault. next day they lock up. the price of mc is £635 not including fitting. go home , source second hand one £40 pick up in rainham 4 hour job. in the garage till 3.30 am . to change it over. hey presto brakes fixed. No stuck on next day. got a mobile mechanic coming on Monday

Oh wait.

Misread "No stuck on next day" (After "brakes fixed") as meaning they didn't stick on the next day, but I suppose the mobile mechanic was a bit of a clue.

Sorry to hear that, but at least you didn't shell out 600 plus notes on a new MC.

Used replacement could be faulty, but pretty unlikely, so looks like its not the MC.

IIRC you've replaced pretty much everything now except the valve suggested above.

Edited by edlithgow on 16/12/2018 at 12:54

they mc ports read S P

Primary and secondary pistons, primary supplies fronts and secondary is the rears, possible the primary piston spring is broken/weak or a seal problem?, will need taking apart to find out.

Had this problem on a Fiesta. Turned out to be a failing front hose which acted occasionally as a one way valve..

I can't see anywhere in the thread what car this is, which might help.

None of my recent cars have had proportioning valves, I think they went out when ABS came in.

So op would need to check if his car has one.

Otherwise you are left with madf's faulty hose theory.

Garages seem to like to clamp brake hoses when working on calipers to save them time.

I would have thought this may well end up damaging the hose internally & may be the cause.

I believe its a Hyundai i10, (doesn't mean much to me) because there's another thread on one with sticky brakes. This thread started as a general brake retraction query but has morphed to replace the fault discussion one which died.

The Internyet is out of control!

I suggested above that an ABS valve might be involved as an alternative to a proportioning valve. I've got very little knowledge and no experience with ABS, (and would like to keep it that way experience-wise) but I believe there are, or can be, valves associated with the MC.

The valves associated with the calipers (2 each, I think) don't seem likely culprits since BOTH rear brakes are reported to stick on.

Similarly, hose failure doesn't seem likely, for the same reason, UNLESS there is only one flexible hose to the rear brakes. This is true of my car because it has a beam rear axle, but I'd think independent suspension would be norm these days

Edited by edlithgow on 17/12/2018 at 12:58

mechanic came today.said discs oscillate which frees off pads, not many people know that.

They usually have anti squeal plates between the piston and the pad or have a special coating to prevent the oscillation going past the pads, even the copper slip used on the steel part of the pad to calliper assists in preventing oscillation which is partly what its for...

is it a solid axle with a single flexible brake pipe to one side, or one flexi per side, as if its a single flexi as has been mentioned it could have collapsed on the inside causing one way valve effect ?

''discs oscillate'', well the mechanic was quite right, first i've heard about this.

Presumably this oscillation stops somehow when the brakes are applied or someone might complain about brake judder.

I don't buy it, but what the heck do i know.

Discs and pads do oscillate but not to the extreme that is mentioned above, the discs are oscillating all the time they rotate which is why the shims are in place behind the pad to absorb it, but as the pads contact the disc the oscillation gets worse, brake judder is the extreme vibration caused between the two

Did you understand my post of Wed 12 Dec 2018 17:37?

I'd suggest you remove the brake servo vacuum pipe and plug the inlet manifold end. Press the brake pedal several times to empty the servo reservoir and then drive around, cautiously, because you'll have no braking assistance and it'll feel like someone's put a brick under the brake pedal.

''discs oscillate'', well the mechanic was quite right, first i've heard about this.

Don't think it is, because I think its another way of saying disk runout, which was discussed above in our wee academic debate on what returns caliper pistons.

Doesn't seem especially likely to be anything to do with the problem though.

I looked up Hyundai i10 a bit. Apparently a main beneficiary of the infamous scrappage scheme, but that's not its fault.

More relevantly, they all have ABS.

Oo-er.

Left and right rear hoses are both advertised for sale on the internyet, which, assuming the sellers aren't full of it, seems to undermine the hose collapse hypothesis.

Edited by edlithgow on 18/12/2018 at 00:40

yes thought that too. will be my next step. might as well try it not a great cost ,

car rolled out of garage this morning so easily

Not sure if that's directed at me.

Just to be clear, my point was,

IF its true that both rear brakes jam on, and IF its true there are hoses both sides, the hoses aren't a single point of failure, so don't fit the symptoms.