A few people seem to struggle with bleeding brakes, especially after replacing a component such as a caliper and are getting a poor brake pedal. I'm hoping this thread might help, it's by no means definitive and can be altered and added to as people see fit.

The main issue is method of bleeding. Normally a problem can occur if the system is allowed to run dry, avoid this as all costs. Block off the hose if possible and safe to do so blanking caps are available, even placing a bar in between the seat and the pedal with a little pressure on the brake system should actually stop air being drawn into the top of the reservoir and thus fluid leaking out. The issue of air getting into the abs block isn't normally a massive one. Valves inside the block should remain open and thus allow fluid to move freely through the block and shouldn't trap any air. However there is an issue if the system has run dry. Most of our ABS systems incorporate EDL, electronic diff lock. A system which applies braking force to a spinning wheel to allow drive to be put through to one with grip. It's a fairly basic system with a solenoid and internal line within the ABS block itself. However this system operates at a pressure of less than 1 bar. If air gets into the block and you attempt to bleed by say pressing the brake pedal the pressure generated is over 1 bar, the valve stays shut and the air stays within. As soon as you start up the abs pump operates the pedal will then sink. The only way to remove the air is a low pressure bleed, something along the lines of a Gunsons EZ bleed should suffice. The ABS block may have to be run to purge the air, this is possible via VCDS for example. This is also why only the front brakes are bled during an ABS bleed using the diag equipment, the operation purges the EDL system, not actually the ABS itself.

Hope this helps a little.

STICKY

Should help most peeps out

Hose Clamping will damage the hose. I know it used to be fairly common practice a few years ago but it's possible under current MOT rules that crushed hoses will result in an MOT fail. Apart from that Hoses are not designed to withstand the severe crushing that is required to block fluid flow . VW/Skoda dealers frown on this practice and if they see crushed hose will recommend that it is immediately replaced for safety reasons.

Ok, I've put in a bit of thought on this. I've seen master cylinder seals reversed (car was non-VAG, non-ABS, and the guy took a very long pedal stroke putting the seals from a worn to an unworn part of the bore).

Accordingly I'd say that seal reversal is unlikely but possible, and suggest the following:-

1) Low pressure (under 1 bar) or vacumn bleed the system.

2) Use VCDS to bleed the ABS block.

If both of these steps in turn do not improve the pedal, then you may have a master cylinder seal reversal.

Thank you for all thread contributions.

In addition to Kens very helpful advice before changing the master cylinder it might be worth bleeding the master cylinder, some have been nipples for this, some you may have to crack a pipe off, worth a shot.

I can highly recommend the gunson ezibleed kit. Got mine for about £13.

Phil

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picking up on what ken has said, i've seen the master cylinder seals get inverted before but never on a vag vehicle... it was caused when the brake caliper piston was pushed back in to change the brake pads which forced the master cylinder seal inside out.. the way i avoid doing this (particularly on vauxhalls) is to crack open the bleed nipple on the caliper when pushing the piston in to prevent the hydraulic pressure build up

picking up on what ken has said, i've seen the master cylinder seals get inverted before but never on a vag vehicle... it was caused when the brake caliper piston was pushed back in to change the brake pads which forced the master cylinder seal inside out.. the way i avoid doing this (particularly on vauxhalls) is to crack open the bleed nipple on the caliper when pushing the piston in to prevent the hydraulic pressure build up

Yeah, MK3 Astras were very prone for that.