Sticky caliper, or maybe not?

[v9]

I’ve just replaced what I thought was a sticking brake caliper, could feel it dragging and it seemed quite slow to release when the pedal pressure was released. Disc and calliper getting hot, usual stuff.
New caliper was fine for a few days and now is doing the exact same. It’s possible that the new caliper is defective, but I doubt this, especially as I’ve inspected the old one and can find nothing wrong with it. All pistons move smoothly.
Can’t figure out what else could cause this problem. Any ideas? Car is a RenaultSport Megane and the calipers are 4 piston Brembo.

[v9]

finlo said:

I've often heard of an internally collapsed flexi hose preventing the return of the fluid though I've yet to encounter it.

Now that’s a thought - do you know a way to diagnose this, or is it just a replace and hope for the best?

[v9]

E-bmw said:

Also, could just be the pads are a bit too tight causing them not to release well.

I normally dress the pads lightly with a file to make sure they move freely when fitted.

Don’t think it’s the pads. I’m leaning towards internal degradation of the flexi hose. It seemed to need more pressure on the brake pedal than I’d usually expect to bleed the new caliper too. That points towards some sort of obstruction I’m thinking. Odd that it seemed OK for a week or so though.

[v9]

GreenV8S said:

Is it affecting one specific caliper, or all calipers?

There's a known 'feature' of common brake master cylinders that the brake circuit is designed to be open to the reservoir when the brakes are fully off. The first bit of pedal travel closes the port between the circuit and the reservoir, turning it into a closed system. If something prevents the master cylinder from travelling back far enough to open that port, the system remains sealed even while the brakes are off. In this state, anything at all that builds up pressure in the system will cause the brakes to drag slightly. That would cause heating within the calipers, leading to fluid expansion, leading to more pressure, and so on in a vicous spiral.

This could be caused by wear or accumulated gunk inside the master cylinder, or just the pushrod to the pedal being adjusted too tight.

To see if this is the problem, try to reproduce the heat build-up problem and then crack a bleed nipple on the affacted wheel. If the circuit is under pressure, either the master cylinder is holding pressure or a brake line has failed and is holding pressure.

If it isn't caused by line pressure then you're looking at a mechanical problem in the affected caliper, or its sliders if applicable.

It’s just the one calliper. Front OS. No sliders to stick as it’s a 4 piston Brembo

[v9]

Shedding said:

How old is your brake fluid?

I had the same on an old XC90. One of the front wheels would get so hot with brake bind that the wheel bolts loosened with thermal cycles. Ultimately, fresh brake fluid fixed it. The old stuff had been in there a long time.

It’s fresh, as I’ve just replaced the calliper, thinking that was the problem. I’m going to replace the hose when a get a minute to do so and see if that helps. It would certainly align with the need for excessive pressure when bleeding if the hose is blocked.

[v9]

Never simple is it? Just got round to replacing the flexi pipe to see if that sorts it. Nut is seized on the hard pipe, so I guess it’ll need a new hard pipe too. That needs half the engine bay dismantled to get access unless I can get a repair on a small section at the end done. That’s probably a job for a workshop as I’ve neither the tools nor the time to sort that at the moment. Just hope I can persuade them to use the flexi pipes I’ve bought. Most places seem very reluctant (with good reason) to say yes to fitting parts they’ve not sourced.

Edited by v9 on Wednesday 24th April 21:02

[v9]

Panamax said:

^^^ This

My car put up a yellow EML light last week but seemed to still be running perfectly. Put it into the garage for a "just in case" OBD check. Oh dear, now that fluttering sound you can hear in the background is a large stack of £50 notes leaving my wallet...

Hope you get the brakes sorted.

Cheers buddy!
Handbrake cables have seized now too. That’s going to need the interior stripping out to get the old ones out I think, oh and the rear discs and pads are on their last legs, and the dampers are getting tired and the top mounts are knocking, and it needs a new PS4 on one corner . . .

Anyone want to buy a lovely RS250?

[v9]

Quick update: replaced the flexi hose and that seems to have fixed it. ‘Twas a right bd of a job though. Had to put a repair on the hard pipe in the end, damaged it trying to get the seized nut loose. Two weeks of soaking in Plusgas achieved nothing!