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Disco brakes, help!

Disco 1, 1997. With ABS. Original fault - brake pedal slowly sinks when heavy pressure applied. No leaks anywhere, no fluid loss. Replaced master cylinder today and then spent 3 hours with a pressure bleeder trying to get the damn thing to bleed!!!! I ended up bleeding to every single union as well as the calipers. One thing that I did notice was the flow of fluid out of the calipers would start reasonably fast then slow right down to a drip, with 10 psi being maintained at the reservoir by the pressure bleeder. Eventually, I got a reasonably firm pedal, but it now has some other weird symptoms, as follows:-

  1. On initial gentle pedal application, the servo seems to almost over-assist, making the braking very sharp.
  2. Servo assistance seems to drop off as the pedal is pressed further, meaning you really have to stand on the pedal hard after the initial bite is over.
  3. Now, when braking really gently, the front left brake doesn't seem to release until you come to a stop and press hard on the pedal then release, then it is fine.
  4. Servo has suddenly become extremely noisy from within the vehicle (air whooshing around). I re-checked the master cylinder to servo seal, it is in place and is the correct one. I checked the master cylinder plunger depth, it is the same as the original one I removed. If anyone out there has any ideas, please help as I haven't much more hair to lose! Thanks all,

Badger.

Reply to
Badger
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I'm not familiar with the Disco 1 but assume that this system is the same as the last of the RRC before they moved to the fully powered system in about 1994/5 i.e. convential dual circuit master cylinder with servo.

In what position has the shuttle valve for the pressure differential switch ended up? Did you unscrew the switch when bleeding?

Assuming that the servo was OK beforehand and that you haven't disturbed it (a reasonable assumption) the whooshing can only be air entering to reduce the vacuum in response to output pressure/output shaft position - it's too late to remind myself how servos work! Probably an indication of something wrong elsewhere rather than a servo problem.

Time for bed....

Reply to
Dougal

Hi Dougal, not familiar witht he RRC system but I think the ABS modulator is similar, that's about it. There is no PDWA and hence no shuttle, just a PRV in the secondary circuit Master cyl output to ABS modulator and Modulator to rear supply. No switch at master cyl. to unscrew whilst bleeding either! Badger.

Reply to
Badger

You learn something new every day - that's a combination of systems that I didn't know existed. It didn't appear on the RRC which only saw ABS with the fully powered system.

I can't help and have no ideas either! But two thoughts: . the bleeding procedure wants the battery to be disconnected. I imagine this is not essential otherwise it suggests that it is disabling a function/feature that would give rise to a permanent battery drain. . do you have the correct replacement master cylinder? The non-ABS one is said to be different but no detail is given: only an external feature for identification. Is it something more than just different springs, for example?

As a totally off-topic comment, I'll be up your way at the end of next week and the beginning of the following - a combination of being en route to the far NW and house hunting in Moray (probably some way to the east of you). Unless you've a major objection and I'm close enough I might drop in on you and show my face. I'll take pot luck on your being at home - no need to advertise any absence here!

Reply to
Dougal

You have mail.

Reply to
Badger

On or around Thu, 6 Aug 2009 23:06:03 +0100, "Badger" enlightened us thusly:

ABS ones are oddities anyway... what type of ABS? I'm assuming you bled it according to the destructions?

Reply to
Austin Shackles

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