Unexpected Brake failure.
 

Hey all.

Have already had a bit of a read up and seen some good advice regarding correct bleed method but I am seeking more a diagnosis is possible by explaining my symptoms.

Long and short of it is the pedal as of today travels all the way down before any braking at all followed by a max 10% of yesterday’s brake pressure.

There was very little fluids in the reservoir so I topped it up today but apart didn’t I haven’t taken callipers off so not given air the chance to get in?

It’s my daily car so I’m gonna be outside until it’s fixed If I can be. Help please.

Well, that's problematic, isn't it?
First step would be to inspect all four calipers to make sure you don't have a leak. I recently had one on one of the rear calipers at the back of the handbrake assembly. Spotting the leak would be easy: just look for dampness. All four calipers should be totally dry. Then inspect the master cylinder to make sure no external leaks are visible.

Also inspect to make sure pads are still intact on all four calipers. This may require shining a flashlight in behind each caliper, etc.

I wonder if you let the fluid get so low that air got into the master cylinder?

It really doesn't take much air for you to experience degradation in pedal feel.

Need more info: had brake work been done recently? How old are pads and rotors?

I would suggest one off your brake pipes has corroded to the point of failure most likely one of the rear pipes

Purchased the car in March but have receipts and visible proof that the rear Caliper had been replaced. The previous owner didn’t service it in 3 years prior to that it’s has fsh from a Volvo from new. I

Did a check today on lines and couldn’t see anything indicating s leak but could t find one. I’m gonna bleed the new caliper tommorow as the nipple will be easy to undo, dubious about the rest but as mentioned could a low resivior result in ait getting in? I thought it was a vacuum?

Cheers for the replies lads

I would have thought low fluid level would result in air in the ststem yes.

You've got a leak somewhere. Brake fluid never needs topping up, it's an entirely closed system. If you've lost fluid it's because it's leaked out. You need to find the leak because it'll jet onto metalwork, and corrode it like nobody's business.

The place to start is the flexible hoses which attach to the calipers. They do burst, had it happen to me a couple of months back.

Easiest way is to look at the floor, the hydraulic oil will form rainbows on wet ground, a puddle of oil on dry ground. That should help you find the leak. Trace back the path of the oil, fix the leak. Wash the oil off any bodywork with copious amounts of water. A garden hose is ideal, but it can be a wet job.

Also a easier to check for leaks if someone is there to press brake pedal while you inspect underneath.

Not at all.
It's fluid pressure which pushes the caliper piston onto the back of the brake pads to maintain braking force.
As the pads wear down, the piston needs to travel further out of the caliper and the brake fluid level will fall accordingly to maintain braking pressure.
Occasional brake fluid top ups can be a normal part of maintenance but are usually unnecessary if the fluid is changed every 2 years.

OP needs to check for pad thickness, caliper operation, air in the system and fluid leaks but low fluid isn't automatically caused by a fluid leak.

Alright, yes you are absolutely correct. I've never actually measured, but I'd expect the relevant slave cylinder displacement to be only marginal compared to the size of the reservoir pot.

Also the level would drop over time and not suddenly all at once, in which case the fluid level warning light would've been flashing on around sharp bends for considerable time. I kinda presumed the OP would've said so if that was the case, so I'm guessing it's all suddenly dropped all at once which would strongly imply a leak :confused_smile:

Nevertheless, I do agree the level will appear to be "consumed" through use, due to gradual wear. I thought it was bad practice to top up this usage? Otherwise when you replace the pads, when the piston is forced back to top, all the extra fluid overflows....?

Anyway thanks for the correction.

To avoid this, you simply attach a tube to the bleed nipple and open the nipple prior to pushing the piston back in.
As the piston is retracted to accommodate the new pads, this will eject the worst fluid out of the system (the caliper fluid) via the bleed nipple and no overflow will occur at the reservoir.
Best practice though is to refresh all the fluid when replacing the pads to give the best operation and avoid having to revisit the area prematurely.