1997 850 electronic brakeforce distribution Topic is solved

Craigd2599 · Post by **Craigd2599** » 09 Oct 2018, 09:31

1997 850 2.4 NA
If I'm sitting at a light or stopped in traffic there will be a "click" and the brake pedal will suddenly depress further toward the firewall. It doesn't do this every time and there are no certain conditions when it happens. Just indiscriminately does this. Otherwise braking is perfectly normal.
Any ideas?

Post by **xHeart** » 09 Oct 2018, 10:44

It sounds hydraulic or vacuum related.

Craigd2599 · Post by **Craigd2599** » 09 Oct 2018, 10:47

Well that narrows it down

Post by **xHeart** » 09 Oct 2018, 10:57

Does CRUISE control works?

Post by **scot850** » 09 Oct 2018, 11:40

There is a vacuum one way valve in the front of the booster housing that fails. Not particularly hard to replace on an NA.

When the pedal drops next time, does the brake pedal go hard or does it just drop to the floor?

Assuming you are not loosing brake fluid. If you were losing fluid but it is not visible, it could then be the master cylinder seals going.

Finally, are all your calipers free to move? If you have one that is sticking (rears are worst) then you might be stopping the car and then the piston frees off and you get the sudden pedal drop. But again in this case I would expect the pedal to be hard initially.

Neil.

Craigd2599 · Post by **Craigd2599** » 09 Oct 2018, 11:49

It does not go straight to the floor. No fluid loss at all. All pistons free. Brakes are fairly recent.

Post by **jimmy57** » 09 Oct 2018, 12:30

Nothing wrong.
EBD, electronic brakeforce distribution, will close rear circuits when a less than ABS worthy speed differential is noted between front and rear. The EBD proportions rear brake by closing inlet to rear and then cycling release valve a few times to reduce the pressure. When you stop and keep firmer pedal pressure, usually on a downgrade at a stop, then when it goes back to normal after few seconds of 0 mph you can feel the click and the pedal will soften and drop a couple of mm. The downgrade or getting caught by a yellow traffic light at the last moment can cause the extra pedal pressure that triggers this feature. That is why it will be intermittent and many will never notice it due to driving style. The 96-98 have greater feel for this with front/rear split master cylinder split. The 99 and later have it but have one rear and one front on each circuit and can absorb the effects better making it less noticeable to driver.

Craigd2599 · Post by **Craigd2599** » 09 Oct 2018, 12:45

Thank you! That sounds exactly like what's going on

Post by **Clemens** » 09 Oct 2018, 15:25

Out of curiosity: do the US 850ies have both front brakes and both rear brakes on separate brake circuits? Ours over here have left front and right rear on one circuit, and right front and left rear on a separate circuit.

Post by **jimmy57** » 09 Oct 2018, 18:27

The diagonal didn't happen to 850 models. S70, V70, C70 on that platform got it in 99. When ABS was introduced everything with ABS had 3 channel with fronts as two different channels (circuits) L and R on the rear port on master cyl and the front port to both rears with one brake line to rear that splits at rear of car. For 99 ABS went 4 channel and the diagonal split was possible again. When your car is up again follow the line to the rear and you'll find a T fitting where L and R brake lines diverge. I looked at parts catalog and I see a rubber line to the T on left delta link and then the right side brake line runs over and has a rubber hose section between the L and R delta links to get to right caliper.