S40 (New) Brakes : My S40 is braking my brain!

[glegalee]

Evening all, the S40 saga continues.

You may have read my previous post about the EPS pump blah blah blah.

I have today replaced the rear coil springs, shocks, lower tie rods and both rear callipers.

I've bled the system thoroughly and have no more air coming out of the bleed nipples however when my wife was in the driver seat as my assistant pressing the brake pedal as per my instructions (press 5 times then hold whilst I release the nipple to allow fluid to flow... repeat) she commented that when she was holding the pedal down before I loosened the nipple that the pedal was slowly moving towards the end of its travel.

I finished bleeding the brakes and jumped into the drivers seat to see what she was on about and sure enough the pedal does indeed slowly continue through it's travel until it hits the end. It doesn't seem to do it every time you press the pedal but there's obviously something not right with the system somewhere.

I've inspected the new calipers and can not see any leaks, there'd have to be a massive leak to allow the pedal to move that much.

I have yet to remove the front wheels and do a thorough inspection at that end but rain stopped play today unfortunately, however I have peeped through the gaps in the alloy wheels and can see the front pipes on which I can not see any evidence fluid leaks.

The reservoir is at the full level and does not appear to drop as if there's a leak in the system.

I'm confused.

Any suggestions would be greatly appreciated.

Cheers Greg.

[IainG]

Hi Greg
Sounds a bit like a master cylinder problem. Sometimes when bleeding brakes the travel of the master cylinder piston is greater than normal. The piston seal can move into an area of the cylinder not normally used. If that area of the cylinder is corroded or rough it can damage the seals.

Especially if no external leaks are showing it has to be internal

[glegalee]

Originally Posted by IainG

Hi Greg
Sounds a bit like a master cylinder problem. Sometimes when bleeding brakes the travel of the master cylinder piston is greater than normal. The piston seal can move into an area of the cylinder not normally used. If that area of the cylinder is corroded or rough it can damage the seals.

Especially if no external leaks are showing it has to be internal

Would the angle of the car cause an issue? The rear of the car was on axel stands whilst the bleeding was taking place and therefore at a slightly steeper angle than in normal use?

Thanks for the info IainG

[IainG]

Hi Greg
I am not an expert but if there was trapped air in the system it would be a spongy pedal, if the pedal goes to the floor and there are no leaks it has to be something internal with fluid leaking past a piston.

Others might have another idea,

[glegalee]

Today the rain held off and I was able to get back out and work on the car. I spoke to one of the neighbours who supposedly used to be a mechanic, he suggested that the entire brake system needed to be bled and that it should be done in a specific sequence. His suggested sequence differed to that in the Haynes manual.

Anyway I enlisted the wife as my assistant again and we set about bleeding the entire system. The neighbour was adamant that bleeding the brakes would fix the sinking pedal issue.

Needless to say that it did not.

I've also been looking in to the idea that the BCM (Brake Control Module) is faulty as when I scan the car with my DIAG system I get an error code...

ECM-C121 - Communication, Brake control modules. Signal missing

I've been looking at my Volvo electrical schematic and have found that the BCM is on the same CAN circuit connections as my bad EPS pump was on which suffered from water (windscreen washer fluid) ingress onto the internal circuit board. I'm wondering if the BCM is indeed faulty and is therefore not functioning correctly and is allowing the brake fluid to pass internally and the noticeable symptom is the slow sinking of the brake pedal when pressed.

The Haynes manual suggests that replacing the BCM is a difficulty level 3 job and should not be overly difficult to carry out. Seemingly the hardest part is getting to the unit as it is bury'd behind the battery tray and plastic covers on the firewall.

Any ideas how the BCM and ABS pump works in this vehicle?

Cheers G.