Brake bleeding, W126

[BillKat]

Hi folks,

Had a sticking caliper which looked to be very crudded up pad pins, and possibly the tin retainer thing battered by a prev owner's attempt to repalce seals. Pistons and calipers were perfect. Flexi line clamped, clingfilm over the reservoir. Cleaned it all up and reassembled. So I went to bleed it yesterday, and after some faffing due to the bleed nipple threads being a bit baggy (vac bleeder), the pedal was feeling very good.
Went to turn the engine on today, and then the pedal went soft and goes much more to the floor .... ?

Should I have bled it with the engine running ? As only the fluid in the caliper was lost that doesn't seem to make sense, but it really is much softer with engine on. I'm confused ...

 

[Ian B Walker]

No need to have the engine running. I would bleed all the brakes again starting from the calliper farthest away from the master cylinder. Don't forget that the lines cross the engine bay to the ABS Unit (if fitted). Also do not press the pedal to the floor when pumping (if that is the method you are using).

 

[BillKat]

Cheers, can't see there should be a need to bleed 'em all though as only one came off, the flexi was clamped, and the res clingfilm'd over. Bled with vacuum bleeder running off a compressor, no pedalling except at the end to test feel. The effect was like a lightswitch: pre-firing up the pedal was good and hard. Soon as we fired up, all soft. I'll have to bleed again, and indeed will prob do the whole system at the wkend. Just can't see why the motor should have such an effect, presumably to do with the booster and/or ABS maybe.

Update, done some surfing, and found folk saying use pressure bleeders rather than vac ones esp if ABS-equipped (though my ABS shouldn't have seen any air), and have the motor running so the booster helps out.

O well, will try again later ...

 

[Dieselman]

If the pedal is firm before starting the engine the problem isn't air in the brakes, just the servo applying assistance which allows the pedal to travel further for the same effort.

I assume fluid isn't being sucked out of the reservoir by the vacuum.

 

[jaymanek]

vac bleeding doesnt always seem to suceed for me... i find best method is with an assistant...

 

[BillKat]

Ta chaps. Bled again, feels the same, so I'll have a careful drive around later and see how it feels on the road. Thanks all.