Brakes never perfect - ABS issue

[paultv]

I thought I'd throw this out there for general information.

I've always had a feeling that the brakes on my cabby were feeling
a bit odd. Despite the fact that they do work ( everything is new )
it always has the suspicion of a pull to the right - more when hot than cold -

I noticed when I fitted a new MC a while back that whilst fluid bled happily through the rears, 20 psi on my pressure bleeder produced no more than a piddle of fluid from the fronts and worst on the nearside.

The only component never replaced was the ABS pump unit, so I took it out and lashed up some lengths of brake pipe with large syringes attached to the ends. Feeding the rear input and output allowed fluid to be pushed in and out of the ABS in both directions with a minimum of effort, whilst the fronts were hard on the off side and near impossible on the nearside feeds, and the nearside return was almost zero unless I activated the solenoid on that side to offer relief.

Anyhoo, I opened up the ABS unit to inspect the filters - this is what I found: The rear input filter was pretty clean and the associated solenoid filter was filthy but fluid could pass. The two front filters were almost totally blocked as you can see from the picture.

So whilst the brakes work, the amount of pressure needed feels higher than it should trying to push fluid through blocked filters,
giving an unresponsive and inefficient system: also if I road test the ABS the N/S would always skid, with no indication that the modulation solenoids were activating at all - all down to blocked fluid pathways.

I've got a replacement pump unit which tests well with my syringe lash up - so now it's time to install it and test, good job its snowing!

http://whitewatermedia.net/media/Close% ... noid-1.jpg

Paul

[DanThe]

Good investigation, I can see a solenoid but no filter

[paultv]

Filter is the cruddy brown area with the plastic cross, crimped into place and not a service item.

Paul

[DanThe]

It looks like solid plastic

[TurtleDesignTech]

Cool investigation

[Brianmoooore]

Very interesting! I'd no idea that there were filters in there. Definitely one to add to the mental database.

[paultv]

If you zoom in you can see the filter structure, but so cruddy it's not serviceable.

I'll report back on the re install of the apparently OK replacement.

I made up an ABS pump and solenoid activator to help with bleeding the pump unit correctly having found a guide on the Bosch website outlining how their
Bleeder plugin functions on later systems.

I simply want to improve my brakes which I believe should be better: I bought a new 525i in 1988 which was both silent and stopped as it should.

Paul

[DanThe]

IIRC the E34 was praised as having one of the best braking systems around at the time

[paultv]

Just a little update on my ABS situation.

Here's a couple of shots of the internals, with a bit of a clean up, prior to re build.

http://whitewatermedia.net/media/input%20filters.jpg
http://whitewatermedia.net/media/input% ... leaned.jpg
http://whitewatermedia.net/media/cleane ... ilters.jpg

First one I call input filters, they feed the solenoid modulator and bypass for normal braking.

Second, how they cleaned up.

Lastly the Modulator filters after cleaning, tricky as they don't come out.

I also managed to clean a lot of crap out of the fluid reservoir but a lot of it is still really
rust stained, there must have been a lot of water in the system over the years.

Paul

[martauto]

A massive case for renewing the brake fluid every two years !!!


Mart.