Brakes: 850: Please help with ODD brake issue

[kurts2]

Hi, I'm new to the forum and not sure if this is the correct place to post as the auto is an 94 but is an 850 model...

Anyway here is the issue. for a few weeks now, the brake pedal will travel low prior to engaging intermittently. It usually happens after driving for around 30 min, and then it may start to work normal again in as little as 5 min. Sometimes it wont work normal again for hours.

Here are the symptoms:
No ABS or brake light.
Pumping will make the pedal grab higher but on next press it goes low again.
It does it with engine on or off (while failing).
There is no pedal fade (except during ABS lock up...see below) even after pumping.
In active ABS braking if the malfunction is present there is a slight difference...done on a dirt road lock up, the pedal seems to engage higher (as normal) but will fade down to lower point with the normal pulsing of the pedal being felt. When the fail isn't active, the pedal stays higher with the pulsing felt. I'm not sure if this is really a fade or just the longer travel combined with normal ABS function though.
Also oddly just prior to the malfunctions occurance, a slight squeal can be heard from one of the wheels...like a dragging brake, the sound can come from either the front or rear...it changes location. A slight press of the pedal seems to confirm its dragging brake. It can clear up or continue until malfunction disappears again.
Last thing of importance, the brakes work well...just low. Pushing hard results in solid braking.

So far have done a bleed and fluid change, no help so changed the master cylinder. That was no help either. Anyone else ever have this problem? I don't want to play guessing games/shotgun maintenance by throwing parts at it. If I was a gambler I'd change the ABS hydraulic module but really don't want to waste the thousands of dollars without some form of diagnostic proof.

Thanks for reading

Kurt

[capt jack]

Start with the basics. How good are your discs and pads? If either are suspect then change the lot. Discs are worn out if there's a discernible lip of a couple of millimetres at the outer edge. Pads should be replaced if they've anything less than 3mm of meat left on them. This can all be checked without even lifting the car or taking a wheel off.

For a closer look get the car up in the air, get the wheels off and inspect the discs and pads, inboard and outboard. You're looking for evidence of wear, or for any evidence of fluid leaking from a calliper.

The 850 series and x70 series cars have a really excellent simple brake set up that really is very DIY-friendly. It's perfectly possible to change the discs and pads on the driveway yourself. All you need is some fairly basic tools - plus of course a good jack and axle stands.

At I guess I'd say that possibly one of your brake callipers has a piston that is sticking. If your brake inspection reveals a pad that is very much less or more worn than all the others then the cause is almost certainly a sticking piston.

You could also take the car to a garage and get them to check the brakes on a brake tester. Many garages will do a basic brake safety test for free.

You can test the servo quite easily yourself. Sit in the car with the engine off. Pump the brake pedal vigorously for a few seconds. It should feel very hard after a few pushes. Then with your foot pushing firmly on the brake pedal start the engine. The pedal should sink slightly as the servo takes effect. Switch the engine off and pump the pedal half a dozen times. The pedal should go very much harder as the vacuum assistance diminishes. This is normal. Anything else isn't.

But my money is on worn discs and pads, and possibly a sticky or leaking piston.

Oh and are you 100% certain that the brakes are fully bled? Is the fluid reservoir topped up? Has the fluid level dropped at all?

Cheers

Jack

[kurts2]

Jack thanks for replying.

To answer your questions, yes the system is fully bled, there are no leaks and the system is full. I just reread my post and realize I didn't make a few things clear. I have done the procedure you mentioned both when its failing and when its not. The results are identical except for how far the brake pedal travels.

That's all this is really about...in a nutshell the pedal travel goes low SOMETIMES. Then it clears up and grabs at the "normal" height. I am about to check the calipers for binding and pad wear issues but I don't understand how they could cause the pedal travel to change from normal to low and back to normal. If they were binding, wouldn't that give less travel not more?

I guess its possible that when the pedal is engaging higher that is the malfunctioning time and when lower its the actual "normal" height.. I wouldn't know as its my daughters daily driver and I only got behind the wheel when she was having problems.

[capt jack]

The state of the discs and pads will I think make a big difference. If one brake was binding, or failing to engage then the car would feel unstable when braking. If a front calliper was sticking then the car would be more difficult to steer when braking.

You've swapped the servo, changed the fluid, and bled the brakes, so really the only things left to check are the discs and pads.

Brakes work by converting the car's kinetic energy to heat, by means of friction. The discs and pads are the physical means by which that happens. It seems reasonable to be that if the two friction surfaces are very worn then they aren't going to do that job as efficiently or as consistently as once they would have done.

A clutch is a similar friction device - and a worn clutch will sometimes slip, sometimes not.

A careful visual check will cost you nothing more than 15 or 20 minutes - it has to be worth it!

Cheers

Jack

[kurts2]

Well it seems problem solved. The rear left outer piston was stuck and the pads were shot all the way around. Fixed all, rebled abd go to go. Thanks for your input Jack.