Soft brakes

[Firemoth]

Technical difficulties.... This car always seems to wait till we're on holiday

Brakes have just gone soft without any warning. They're still working but less effective. I've had a quick check and there are no visible leaks and no loss of fluid. I suspect it could be the rear brake compensator, although I'm not sure exactly how it works. I do know that since I got the car a year ago, if it's parked up for a while, for the first mile or so, the rear brakes are very very keen. Very easy to lock the back wheels up, although it's completely fine after s few stops, so it makes sense it could be that.

We've just got to where we're going (fortunately) so it will be a few hours till I'm back at the car. If anyone has any suggestions what to check, that would be helpful.

Thanks

1997 synergie TD (drums on the rear)

[xantia_v6]

Is the pedal spongy (does it move further than usual), or has the pedal gone hard? I am not sure from your description above.

[Stickyfinger]

If you need it checked quickly take it into a garage.
Brakes are not the thing to learn about just before a family holiday.
The system is simple so should not cost much to sort out.

[Firemoth]

Is the pedal spongy (does it move further than usual), or has the pedal gone hard? I am not sure from your description above.

The pedal has gone soft, suddenly

If you need it checked quickly take it into a garage.
Brakes are not the thing to learn about just before a family holiday.
The system is simple so should not cost much to sort out.

We're already away. I have my tools with me, and I am competent, I'm just asking for suggestions. I'm not a fully fledged mechanic but I'm not a total novice. I'd rather ask with things I'm not completely familiar with than just randomly guess. If I don't feel comfortable doing something, I wouldn't do it

[Stickyfinger]

Ok, no problem with that.

First job then would be to take the rear wheels/hubs off and see what is there, report back. Without that information you could go down many time wasting roads.

When did you last check the rear brakes ?

[Firemoth]

Update, back to the car now, it's been sat 3 or 4 hours, brakes are completely fine now. I'm going to suspect mainly the master cylinder, so I'll replace that when I get home just in case, and probably to the compensator valve too because that has been odd as mentioned. I think maybe it's unlikely to be that but can't hurt. Fingers crossed all will be ok till I get home.

There is no wet from external leaks, and obviously an external leak wouldn't "fix" itself. I doubt Flexi hoses too. I've had a look and can't see any issues

[white exec]

I take it you have bled the brakes?
Trapped air will produce spinginess and a soft pedal, which can firm up with several presses of the pedal.

If standard DOT4/5 brake fluid, and this hasn't been renewed in years, possible that it has absorbed moisture - it's fiercely hygroscopic. This trapped water vapour can turn to vapour if the brakes get really hot, and produce fade/sponginess at the calipers/pistons.

Before dismantling, renew the fluid, and bleed the system. Should be easy to do.

[Firemoth]

It's not air in the pipes. If there was air it wouldn't be sudden and they wouldn't have felt right previously. I've just travelled 300 miles with no issue at all. There are no leaks, no bulging flexis or anything. Fluid level hasn't moved. I suspect probably the master is giving way internally, but I don't know if the compensator valve could play any part.

The fluid is less than a year old, I flushed that maybe 10 months ago when I replaced the front pads and discs

[moizeau]

Does the synergie use the hydraulic pump for the steering and brakes only being a coiler like the ZX? If so how's the steering when the brakes are playing up? Does the pedal feel the same as when the engine isn't running?
Just a thought, possibly cobblers..

[Sharkie]

It may be brakebooster / master cylinder.
Brake booster leaking will cause pedal to get hard or soft depends if you close the leak when you press down. Test it by turning off your engine and pumping brakes till they are hard. When you turn on the engine it should sink in.

If it's master cylinder, just turn engine off and pump it hard. Then hold the pedal down, it should not move. If it sinks, it's the master cylinder (or well, seals in it). Costs about 20 "units of currency" at your local stealer. Get a new one, they are cheap and old ones from scrappie leak and are crap.
Seals bend, roll over, etc. randomly or just rip when you press pedal hard. they are old, 25yrs now, well past design life.
It is also very .. occassional as there's a flat spot on piston, somewhere it seals ok, when you press further it leaks thru.

And yeah, changing fluid ruined it. Gunk was sealing it. don't worry, you can always fix it with more paper bank notes.

[Gibbo2286]

The first thing I would be looking at if this was a sudden occurrence would be the rear brake self adjusters, they often broke and let the adjustment fall right back to the stop.

[Firemoth]

Update, something like 1500 miles later, from Poland, through Slovakia, Austria, Germany, Luxembourg, across Belgium into France and back home, plus another couple hundred miles this weekend, absolutely nothing wrong. Can't find anything at all wrong. I can only think it's that compensator or the master cylinder. I'm planning on doing the rear shoes and cylinders soon anyway, so I'll do them two too just in case. Weird