Hissing brake pedal Citroën XM

[TheDaveeeder]

Hi!

I'm new to the french car world and am getting used to the funny hydraulics. It understand that I can't diagnose a brake pedal the same way as I'm used to, and so I am asking all of you professionals for help!

----
My XM 1997 has a weird brake pedal. The pedal hisses when pressed with the engine on, like a bad brake booster, but I understand a brake booster isn't needed in this car?

The pedal is also spongey, and can with a bit of effort be pressed to the floor.

While driving at high speeds, the brakes work just fine. But at low/very low speeds, the brakes work very poorly, and when pressed slowly, hard vibrations appear in the pedal as if the disks slip because of not being pressed hard enough. And if the brakes are then pressed to the floor (trying not to hit the car in front of me because of the bad brakes ) there is a delay of around 3 seconds before the brakes work with full power ("as if "I was slamming on the brakes ).
----

I should add that the front brakes were renovated with newly greased guide pins, new discs and pads. The brakes were bled in the correct order, with the rear brakes having much air and nothing in the front. After this, nothing with the braking had changed. The brakes were bled once again, and the rear had about 1/4 of the amount of air as the first time.

There is also a LHM leak in the front. Not major, but it's definitely greasy.

Thank you very much in advance for all help, it is very much needed!

[CitroJim]

My first check here would be on the condition of the main accumulator sphere... Such symptoms as you have suggest it's flat...

I've had exactly this due to a below-par accumulator.

Or there's possibly insufficient hydraulic pressure ay low engine speeds to effectively operate the brakes - this will be magnified if the accumulator is flat or ruptured.

With the engine idling, listen for a 'tick' from the pressure regulator mounted on the front of the engine by the gearbox. On top of the regulator is the accumulator sphere.

If the ticks you hear are anything much less then 30 seconds apart, the accumulator is suspect. Rapid ticks or none at all indicate either a ruptured accumulator, a duff pump or faulty pressure regulator...

Else you have a serious internal pressure leak.

Were all brakes bled correctly? The rears need a special technique and at least 500mL of LHM needs to be bled out each side to ensure they're properly bled.

[TheDaveeeder]

My first check here would be on the condition of the main accumulator sphere... Such symptoms as you have suggest it's flat...

I've had exactly this due to a below-par accumulator.

Or there's possibly insufficient hydraulic pressure ay low engine speeds to effectively operate the brakes - this will be magnified if the accumulator is flat or ruptured.

With the engine idling, listen for a 'tick' from the pressure regulator mounted on the front of the engine by the gearbox. On top of the regulator is the accumulator sphere.

If the ticks you hear are anything much less then 30 seconds apart, the accumulator is suspect. Rapid ticks or none at all indicate either a ruptured accumulator, a duff pump or faulty pressure regulator...

Else you have a serious internal pressure leak.

Were all brakes bled correctly? The rears need a special technique and at least 500mL of LHM needs to be bled out each side to ensure they're properly bled.

Thank you very much for the leads!! I will have to listen for ticks.

About the brake bleeding, that was not something I had gotten out of the Haynes manual. Could you maybe link those instructions or so?

[Hell Razor5543]

Brake bleeding on green blooded Citroens is not that difficult, but will HIGHLY benefit from some work done in advance. It is not uncommon for the bleed nipples to shear off if they have started to rust. To reduce the chance of this happening spray some 'Plus Gas' onto the bleed nipples every day for a week or so in advance. This should soak in and free off the bleed nipples.

You may need to get 1l LHM, as you will not know the state of the LHM fluid in the brake lines. If the LHM reservoir is on the left of the engine bay (standing in front of the car) the bleed order would be (from the perspective of sitting in the driver seat) rear left, rear right, front left, front right. This starts with the brake furthest from the reservoir, ending with the brake closest. If you have long enough clear flexible piping you can run it from the brakes to the reservoir, otherwise use clear pipe into a large jar.

With the engine running, the handbrake on and wheels chocked, raise the car to full height. Place a heavy weight onto the brake pedal (anything that will keep it pressed down will do), remove each wheel in turn, loosen the bleed nipple and bleed the brake until there are no bubbles (if thee are a lot of bubbles it is possible a sphere has failed). Tighten up the bleed nipple (but not over-tight), fit the wheel, and do the next wheel.

I hope this helps.

[TheDaveeeder]

Brake bleeding on green blooded Citroens is not that difficult, but will HIGHLY benefit from some work done in advance.

Thank you for the tips.

My first check here would be on the condition of the main accumulator sphere... Such symptoms as you have suggest it's flat...

With the engine idling, listen for a 'tick' from the pressure regulator mounted on the front of the engine by the gearbox. On top of the regulator is the accumulator sphere.

If the ticks you hear are anything much less then 30 seconds apart, the accumulator is suspect. Rapid ticks or none at all indicate either a ruptured accumulator, a duff pump or faulty pressure regulator...

What do you think of this? Are those very rapid ticks?

Also, I *think* I hear a vacuum leak somewhere to the right of the sphere. Could this have something to do with anything?

[xantia_v6]

There are few items activated by vacuum on an XM, only the waste gate on a turbocharger (if fitted) and the cruise control (if fitted).

A hiss from the hydraulic regulator (the thing that the accumulator sphere is mounted on) may be due to the pressure bleed valve not being properly sealed. That could be due to the 12mm bolt not being tight, or someone may have removed that bolt and lost the ball bearing that sits behind it. These would also cause fast tick rate.

A hiss from the brake valve is not usually of concern, without other symptoms, and may be due to wear inside the valve, but there is no benefit in trying to repair the valve for that reason.

[TheDaveeeder]

There are few items activated by vacuum on an XM, only the waste gate on a turbocharger (if fitted) and the cruise control (if fitted).

Thanks, good to know!

A hiss from the hydraulic regulator (the thing that the accumulator sphere is mounted on) may be due to the pressure bleed valve not being properly sealed. That could be due to the 12mm bolt not being tight, or someone may have removed that bolt and lost the ball bearing that sits behind it. These would also cause fast tick rate.

Okay, I will check this.

A hiss from the brake valve is not usually of concern, without other symptoms, and may be due to wear inside the valve, but there is no benefit in trying to repair the valve for that reason.

That's good to hear!

I have 4 new spheres laying around. What do you think of replacing them to begin with? Two of them are febi 22503 and two are 23017. Which ones of the 6 spheres would you say to replace?

[xantia_v6]

Febi 22503 appears to be a rear center sphere.

Febi 230176 appears to be a main accumulator sphere.

[TheDaveeeder]

Update: It's the ABS that's the culprit!!

With the fuse removed, the brakes work perfectly fine (except for the ABS, which is deactivated and has a lit warning light). However, with the fuse back in, the ABS behaves very strangely (and not to mention dangerously) at low speeds, as explained before.

There's a lot of DIY electrical work and many cut cables, so I guess I'll have to start dismantling things above the ABS to check if the cables are connected properly. I tested the relay (I believe it's the dark green one labeled R6H920) and the coil/contact seemed to work. There was around 80 ohms between pin 30 and 87 when activated, and nothing when the switch was open. However, I couldn't get a clear measurement between 30a (the small contact with a diode in the circuit) and 87. I'm not sure what the reading should be between them.

The frustrating part is that I got a bunch of relays that lack the 30a contact, and I tested with one of those new relays, but it was exactly the same as with the original one. Hence, I'm not sure if the 30a contact is faulty on the original relay.

Does anyone know where I can get such a relay if it might be worth replacing anyway? The original part number seems to be faded. I couldn't find much about R6H920.

Does anyone have a better understanding of the system than I do? Are there other points where things could go wrong besides the ABS unit or the electronics there? I'm also thinking it's odd that the ABS warning light doesn't come on even though the ABS isn't working. Could there be an issue with something that no sensor is reading?

[TheDaveeeder]

Now the brakes and ABS work perfectly! Thanks for all your help.

The ABS sensors were positioned too far from the rings. I ground down the contact surface between the sensors and the wheel hub and attached them so that there's only a millimeter gap between the sensor's tip and the ring. The pulsating and bad brakes at slow speeds was because of the sensors to getting sufficient magnetic pulses from the rings.

[CitroJim]

Excellent

[Armidillo]

Well done - a great bit of detective work!