Im having a strange issue with my 2010 mk1 octavia pd100, i replaced both rear callipers and bled the brakes and mc, since then the brakes are fully released when the engine is off and the car has sat for some time, but the second i apply the brakes they dont fully release. I have tried disconnecting the servo vacuum and this also doesn't release them the only way I can get them to release is to release some fluid from the MC bleed valves. Just wondering if this is going to be a MC fault as im at a loss right now, thanks

Edited 7 July, 20257 Jul by illusive

At first look I'd suggest you either have trapped air or spongy flex hoses.

Would the flex hoses be likely to fail along side bleeding the brakes? Was never and issue until I changed the callipers, definitely no air in the system full bleed along side vcds abs bleed

Edited 7 July, 20257 Jul by illusive

Why would air in the lines cause the calipers to not release ?

To not release means either the cakipers are binding or the brake pressure can't release in the system. If anything air in the system would help the caliper pressure release.

You say opening a bleed valve releases the pressure so I agree with you in thinking the master cylinder is not letting the pressure release back into the reservoir.

I assume nothing is preventing the brake pedal from fully retracting and providing a fully released pedal state to the master cylinder. In that fully released state the master cylinder should just let fluid return to the brake reservoir. Like when you use a wind back tool on the brake caliper to change pads. The fluid should rise in the brake reservoir as the calipers are wound back, the master cylinder letting the fluid flow backwards. From what you describe that would not be occurring.

8 hours ago, aubrey said:

Why would air in the lines cause the calipers to not release ?

Anything elastic in the hydraulic path from brake pedal to caliper piston can cause sponginess. If the elastic element is pressurised by action of the brake pedal the pressure will continue to act on the system - including the calipers - when the pedal pressure is released. Think of blowing up a balloon.

I cannot recall now whether the dual brake system is parallelled off the back of the master cylinder or downstream but both sides share the same pressure - there is only one master cylinder with shared dual lines and IIRC no non-return valves in the brake lines. This means that a pressure anomaly anywhere in the braking system would be felt by all the brake elements.

Just FYI I had a complete brake failure on my mk1 at one point, fortunately only at walking pace in a Morrisons' car park. Turned out the master cylinder seal has failed which allowed the brakes to pump our fluid from both circuits as I pumped the pedal looking for some stoppage. Officially this is not supposed to happen with a dual braking system - but it did :(

A detail i seem to have left out now that im reading back over the post is that the pedal is far from spongy in fact its almost like a rock once the pressure builds in the MC, whatever is causing them to bind is binding all 4 wheels equally from what I can tell and anywhere in the system i bleed lets out a smooth flow of brake fluid with 0 air. Im really just trying to see if Ill get away with changing just the MC or if I broke something in the servo causing the piston to be extended too far in MC if thats even possible. Ive checked for obstruction at the pedal and it is fully up nothing stopping travel on the inside of the car anyway. And if the car is parked for a while the pressure in the MC dissipates allowing the brakes to free up again 🤷‍♂️

wind the caliper pistons back fully if they press back inwards without too much force this should show calipers and the brake hose have no air

2) make sure the brake resovoir cap is off

dont forget secure it back on

also just as a safety measure open the bleed nipple gently clean it first then libricate it and then slack it off see if air bubbles come out with the brake fluid

this is what I would try first

15 hours ago, illusive said:

Im having a strange issue with my 2010 mk1 octavia pd100, i replaced both rear callipers and bled the brakes and mc, since then the brakes are fully released when the engine is off and the car has sat for some time, but the second i apply the brakes they dont fully release. I have tried disconnecting the servo vacuum and this also doesn't release them the only way I can get them to release is to release some fluid from the MC bleed valves. Just wondering if this is going to be a MC fault as im at a loss right now, thanks

This might seem off the wall, but do you get similar symptoms with the clutch?

Yep the callipers all wind in with no force, thats the first thing i tested and no clutch is still working a treat

once you tried and pushed calipers back easy when putting back toghether did you replenish the brakefluid

also worth noting to use a manual pressurised bottle system from halfords for brake fluid replenish works a treat

does the brake pedal feel low or is it depressing ok after dont worry about soft pedal

pump hard but gently until you see its all satisfactory recheck for leaks brake hoses nipples etc should be air has vapourised and brakes bleed should be successful once your brake hose are also in good condition

I have another car very old 23 years old toyota and needs new brake hoses and full pressurised brake fluid bleed never been done and key fact car has been stud battery went flat and renewed similar issue to yours irony is do not leave your cars sat for days weeks months get in and drive them as things like brakes cease up etc

once you tried and pushed calipers back easy when putting back toghether did you replenish the brakefluid

also worth noting to use a manual pressurised bottle system from halfords for brake fluid replenish works a treat

does the brake pedal feel low or is it depressing ok after dont worry about soft pedal

pump hard but gently until you see its all satisfactory recheck for leaks brake hoses nipples etc should be air has vapourised and brakes bleed should be successful once your brake hose are also in good condition

I have another car very old 23 years old toyota and needs new brake hoses and full pressurisedf brake fluid bleed never been done and key fact car has been stud battery went flat and renewed similar issue to yours irony is do not leave your cars sat for days weeks months get in and drive them as things like brakes cease up etc

one more last thing i dont know if uour skoda is rear brakes drums or brake discs and caliper

if drums check the cylinders inside for leaks anything that shows oil stains

common on drum brakes

lastly clean and regrease and refit the brake caliper pins this can lead to brakes binding had this problem

on my avensis

I reckon it has to be something to do with master cylinder if all brakes are the same. Had a similar problem with one brake due to a colapsed flexi but all four the chances are nil.

Alasdair