Rebuilding front brake calipers - Audi Allroad

[old_coyote_pedaller]

tis true. And with the cap on the resovoir, its like a thumb over the end of a straw filled with water

G

Just to be extra sure I put a bit of poly-bag over reservoir before putting cap back on. Had one with a vent where fluid still leaked out.

 

[drcarlos]

Opening the bleed nipple won't require the brakes to be bled unless the brake pedal has been pressed and released with the nipple open allowing air to be drawn in.
If you don't do this how is the air going to get in? For air to get in fluid must come out for the air to replace it in system. Crack open the bleed nipple and you will find that fluid weeps out, the weight of the fluid in the pipes above caliper pushing it out, leave it open and you will soon have an empty fluid reservoir. How do I know that?.....

tis true. And with the cap on the resovoir, its like a thumb over the end of a straw filled with water

G

Not necessarily a fluid by it's weight can create negative pressure as there is already air and space at the top of the system in the reservoir, when you open a bleed nipple to atmosphere you take a chance that some fluid will not dribble out as you mess around compressing the piston which may move back out slightly drawing air in.

Try it with a straw that is half full of air at the top.

By the systems design the fluid must be able to flow into the system with the cap on otherwise it would not self adjust and pedal travel would increase as the pads wore (just like maguras).

Carl.

 

[old_coyote_pedaller]

Not necessarily a fluid by it's weight can create negative pressure as there is already air and space at the top of the system in the reservoir, when you open a bleed nipple to atmosphere you take a chance that some fluid will not dribble out as you mess around compressing the piston which may move back out slightly drawing air in.

Try it with a straw that is half full of air at the top.

By the systems design the fluid must be able to flow into the system with the cap on otherwise it would not self adjust and pedal travel would increase as the pads wore (just like maguras).

Carl.

That's the whole point of putting a plastic pipe on nipple so that the fluid goes into pipe and will be drawn back in instead of air. For air to be drawn in there would need to be a negative pressure in system as you opened nipple. Even if that was possible, taking the cap off reservoir would release the pressure. If you have negative pressure in system there's something wrong other than a sticky piston.

"By the systems design the fluid must be able to flow into the system with the cap on otherwise it would not self adjust.."

Yes but it flows from reservoir down as pads wear, the only way it's going to flow other way is if pistons are pushed back with nipple closed.

More than once I've removed and replaced a caliper and found that with bleed nipple open as I reconnected caliper that it basically self-bled.
I do not recommend anyone do this as a work method, it's always safer to bleed the caliper after refitting.

 

[unkleGsif]

Not necessarily a fluid by it's weight can create negative pressure as there is already air and space at the top of the system in the reservoir, when you open a bleed nipple to atmosphere you take a chance that some fluid will not dribble out as you mess around compressing the piston which may move back out slightly drawing air in.

Try it with a straw that is half full of air at the top.

By the systems design the fluid must be able to flow into the system with the cap on otherwise it would not self adjust and pedal travel would increase as the pads wore (just like maguras).

Carl.

But air will not be drawn back into the system purely from the wieght of the fluid, unless the pedal is pressed and allowed to rise without the nipple being closed

Listen. We are arguing the toss here. We both know that air in the system must be bled out, all i was originally saying is that for a few seconds, to avoid a major fluid spill down the most inaccesible places of the car, and with a little care, its easy to just crack open the bleed nipple and let the excess fluid drain where you can see it. If you dont do anything stupid, then there is no problem


You dont want to know how Ive seen some people dleed brakes in a garage that I "USED" to work at.... bottle of dirty fluid used from one car to the next, pipe from the open nipple and just pump the shit out of the brake pedal till it feels OK :shock:

G

 

[old_coyote_pedaller]

I believe you.
I wouldn't trust a garage to even put air in my tyres after I bought a car from one and wondered why it was handling so weirdly, checked tyre pressures, they were all 60psi.

 

[drcarlos]

But air will not be drawn back into the system purely from the wieght of the fluid, unless the pedal is pressed and allowed to rise without the nipple being closed

Listen. We are arguing the toss here. We both know that air in the system must be bled out, all i was originally saying is that for a few seconds, to avoid a major fluid spill down the most inaccesible places of the car, and with a little care, its easy to just crack open the bleed nipple and let the excess fluid drain where you can see it. If you dont do anything stupid, then there is no problem


You dont want to know how Ive seen some people dleed brakes in a garage that I "USED" to work at.... bottle of dirty fluid used from one car to the next, pipe from the open nipple and just pump the shit out of the brake pedal till it feels OK :shock:

G

Wasn't what I was getting at, just difficult to explain on here with a 1000 word essay, but yes I agree a quick bleed every time with my one man kit is what I do with a bit of fresh fluid.

Carl.

 

[ere-tis]

Brake fluid gets dirty over time (the cr@p that seizes up your calliper pistons works back past the seals etc) as the fluid doesn't circulate the system the dirty fluid 'lives' in the calliper region of the brake line.

Force the piston back without loosening the bleed nipple and you'll push all the crud back into the ABS module.

The module seals are incredibly delicate and not replaceable, so new module time I'm afraid.

I have to admit I thought about this a fair bit since I read it and I would suggest it's FUD spread by dealerships/manufacturers trying to scare people into having an easy home job done by a garage.

In my case, it comes directly from experience.

I was that man that forced the pistons back without releasing the bleed nipple, I was the man who's abs went tits up within minutes of the 1st drive.

We tried all sorts including power bleeding the system and even energising the abs pump with a tech 2 handset to make sure all the air was expelled from the system.

I managed to pick up a known good 2nd hand abs module (new one was £1100) fitted that and bled the system and all was fine.

A very good friend of mine is a Vauxhall Technician and he performed a post mortem on my module and found crud and rubber shavings in it.

He has a very good friend who he did his apprenticeship with who now works for Bosch, he then stripped the module and confirmed dirty fluid being forced back into the module fubared the seals.

I'm not a brake specialist myself, but I'd trust my mate with my life and if he takes the word of a specialist who made no financial gain from my problem whatsoever, I'm as sure as I can be what I posted is 100% accurate.

 

[Supratada]

I believe you.
I wouldn't trust a garage to even put air in my tyres after I bought a car from one and wondered why it was handling so weirdly, checked tyre pressures, they were all 60psi.

As I understand it, garages sometimes overinflate to prevent flatspotting if the car is sitting on a forecourt for a while. ( I did it when I laid my car up for 18 months) Of course, it should have been picked up in the PDI.
Ooh I love a good acronym.