Brake pedal soft but brakes fine. Doesn't pump up, no fluid leak - air in system or master cylinder problem?

Hello,

Sorry if this is a tired topic, but I don't really understand it properly. I fell into the trap of consulting AI on it a lot back in May as it would give quick responses to all the details, and at the time it seemed like it was realistic and made sense, but really it doesn't seem to track.

So originally my MK1 Fabia had a hard pedal. After a wild goose chase with the servo and its vacuum pipe (which would have become a problem later anyway), we found that the actual cause was a blockage in the ABS pump causing 1 wheel to receive no brake fluid. So we replaced the ABS pump and the pedal was a lot softer and we had much better braking performance.

We bled the brakes a few times and replaced the front brake pads as my dad wanted to change them. Then we ended up bleeding them again as we noticed that the pedal was a bit soft. I don't remember at what point we first noticed this, and if any difference was made by the brake pads. It is possible that it was always that soft, but we didn't notice initially as we were expecting a much softer pedal after fixing the old hard pedal.

So my Uncle who is a car mechanic who we asked about it, insisted that the brakes were soft because of the new pads and I was just overblowing it because I'm a snowflake or whatever. But I can't see that it would make them that soft, and 1000 miles later the pads should be worn in but there isn't any difference.

So originally we assumed it was air in the system and we bled it a load more, but we don't know if there is air in the pump that can only be bled by doing a VCDS bleed. But surely the air trapped in the pump that a VCDS bleed would get out, would not be part of the normal braking hydraulic circuit, until ABS kicks in and extra valves open and the pump has to operate. Otherwise you would be able to bleed it normally and only have the braking dynamics change after an emergency stop where extra air is introduced to the normal circuit. But then I looked more into what I heard about how you are not supposed to use the full pedal travel when doing a 2 man bleed. My dad said he didn't think this was necessary as he had never had that problem with another car. So the AI ended up convincing me that I had damaged the master cylinder seals by bleeding it improperly, which I came to accept, but I haven't yet bought the parts for that job as I keep putting it off as I'm anxious about bleeding the clutch and that becoming a similarly long story. I have already determined which master cylinder and servo mounting gasket I need for that. I just haven't ordered anything.

So I've been driving it like this for about 1000 miles (admittedly a lot of them motorway/dual carriageway, but not exclusively) and it stops perfectly fine, just the pedal travel is obviously more than it is supposed to be. I've kind of got used to this now. My dad thinks it might fail its MOT, which is on Friday, for insufficient reserve pedal travel. So I've been told to get the new master cylinder ordered in case we need to change it for a re-test.

But today I am looking at other posts on this topic, and it sounds like when the master cylinder seals fail, this is normally accompanied by fluid leaking from the master cylinder either into the engine bay or into the servo. To my knowledge, neither of these have been occurring and the fluid level has not gone down significantly since we last messed with it. I would expect it to go down a bit due to the brakes bedding in. I need to check this again though in the light tomorrow. I hadn't been looking for leaks there before, only at the ABS pump unions.

So now I am wondering whether it is the seals after all, or is air in the system. How do I tell?

The brakes don't "pump up", which is why I didn't think it was air in the system. Instead, you get a bit of travel, then the pedal gets harder, then if you press on the pedal for a longer time, it very gradually sinks. You can only really do that if the engine is running and the servo is active. The pedal doesn't actually hit the floor, but I don't know if that is possible or if there is a stop somewhere that another part of the pedal hits, or in the MC. So that seems like the MC seals, but why then are you still able to get good braking pressure, and why is there no fluid loss?

The master cylinder is the 19 FTE 5615 one if it makes a difference. And the ABS pump is the mid range one that has ASR but not ESP, I think, though I can't remember exactly. Mine doesn't have the ASR button but I think that's because it's an '07 and you can't turn it off. It definitely isn't the ESP one though as we didn't need to do the steering angle calibration when we replaced it.

Sorry I've made this really long winded. I'm just trying to include as much information as possible that might be relevant.

Elliott