My brakes work fine in normal town and country driving, easily able to stop even with harsh braking. With heavy sustained braking, the pedal moves beyond normal limits so much that my foot is pressing on the accelerator and brake pedals simultaneously. Total travel on the brake pedal is more than 50%.

Anyone have any thoughts?

 

Edit:

I should have added that when I try pressing hard on the brake pedal with the car stationary and engine running, the pedal gradually moves down under constant brake pressure.

Edited 12 October, 20214 yr by pikpilot
Additional info added.

When was the brake fluid last changed? Had it it tested even for h20 content.  Has there been any inspection of the brakes and any servicing in recent times?

Brake fluid changed early December 2019. I have asked them to do another brake fluid change today.

It is serviced by trusted local Skoda garage. Their only recommendation was to change the front disks and pads "as there is a lot of rust on the outer part the disks".  I said no for now as the rust is outside the swept area of the pads. They checked the pads and disks as 20% worn in Dec 20. Due to lock down the car has only done 1600 miles since then.

 

I see from the last brake fluid change invoice that they only added 1litre of fluid. Sounds a bit small?

They have a special VAG machine for doing brake fluid changes.

check the master cylinder above the brake pedal if there's any fluid about its leaking 

7 minutes ago, lukegcorrida said:

check the master cylinder above the brake pedal if there's any fluid about its leaking 

 

Checked by myself and the garage, no sign of leaking brake fluid. This was my first thought too. Other possibility suggested on-line is the seals in the master cylinder were shot but no way to easily check without replacing it. 

 

Just got the car back from having the brake fluid changed. I notice now there is less movement on the pedal before it hardens up and starts to work. On heavy braking (car stationary) there is less sink but still the pedal falls below the level of the accelerator so I still have the engine fighting the brakes. Perhaps I need narrower shoes and feet!! 

Try brake pedal travel before and after fully applying the handbrake, this may at least indicate whether the problem is at the front or rear.

Before running my last series of tests, I pressed reasonably hard on the brake pedal while pulling up the handbrake in order to reset the auto adjusters. It also gives almost no movement on the rear pads before they touch the discs.

This gave me minimum "slack" pedal movement before the pads touch on the front discs and a consistent starting point.

 

Having done what many people do and looked for answers on the net, the only answer that come close is a failing master cylinder. The Skoda garage looked at the possibility when trying to diagnose the fault but ruled it out because there is no easy way to confirm this as the cause and secondly they say they have never ever had to replace a faulty master cylinder. 

 

The car is going in soon for the MOT and service so that Is why I agreed to pay for a brake fluid change as a first step. (It was due anyway at the next service).

 

Thanks MicMac for your suggestion. My thought is that it is at the front and if not the master cylinder the perhaps it is in the antilock brake system.

(In the past it was found that air could be trapped in the antilock brake valves unless the exercised using software during brake bleeding. The garage ruled this out as they now use a VAG specialised brake bleeding machine which takes care of this. - Anyone know anything about this machine? Perhaps something like this https://garagewire.co.uk/news/company/sykes-pickavant/new-electric-brake-bleeder-at-sykes-pickavant/ )

 

When purging the ABS block, the brake calipers' bleed nipples are systematically and manually opened/closed in accordance to a device connected to the car's OBD port which activates the ABS.  Typically the brake fluid reservoir is positively pressured with fresh fluid or the calipers' bleed nipples are drawn via a negative pressure fluid collector, thereby evacuating the system of old fluid and any trapped air.

 

The machine you linked to is simply a fluid pump to bleed the brake system, possibly with enough pressure/flow to eliminate the necessity to activate the ABS electronically but from what I've seen, problem systems generally have the ABS purged electronically in conjunction with similar machines to ensure no trapped air.

Edited 24 October, 20214 yr by MicMac

Thanks for the information. I will use this when I see the workshop leader. I remember the need to activate the ABS through the diagnostic port on the Mk 1 Octavia.

 

I had similar issues recently after removing a rear caliper, putting it back and bleeding the brakes - the pedal was solid with the engine off, but gave way slowly when the engine was running. I too searched for answers, and found a video where the solution had been to activate the ABS (skidding on a gravel patch).

I tried it, not sure if it worked, but i didn't seem to feel I had a problem with my brakes after that.

Thanks for the comment. I will give it a try.