Changing Front Brake Pads on P Reg Astra.

I used to just push them back until I once screwed up a caliper..:o(

So now I use a small piece of fish tank air line attached to the bleed nipple leading int a bottle, slack off the nipple and the piston easily pushes back into it's housing. Also, no danger of air bleeding back via the nipple with this method.

PDH

Just curious. Why is there a possibility of screwing up the caliper seals if the pistons are just pushed back on an Astra? AFAIK it's the conventional way to replace pads. Mike.

Yes.

Never needed to do this.

Yes. Because you've removed the lid from the reservoir they should push back easily without the need for nipple tweakage.

Never needed to do this either, just a good push on the pedal afterwards to settle the pistons back where they want to be.

Si

I think it's one of those "may as well since I'm here" jobs. No harm, if done correctly.

Some brake systems with ABS do not like the brake caliper piston compressed without the nipple released and it has been recommended in the past to undo the bleed nipple to wind back the piston rather than take the risk of damage to the ABS servo.

To be honest, I prefer to do this method on non ABS cars as well, any dirt collected in the caliper can be ejected rather than forced back up the line and there is the added advantage of topping off with fresh brake fluid.

PDH

The message from "Mungo \"Two Sheds\" Toadfoot" contains these words:

The advantage of releasing the nipples is that the dirty grotty fluid comes out of the caliper and you get the chance to top up with nice clean fluid at the top.

Doesn't overflow, either.

And keeps the nipples free instead of seizing if you smear a little copper grease on the thread before replacing 'em.

Oh right, furry muff then. I've never owned one of them new-fangled cars with ABS and stuff :)

Si

I'm convinced.

Si

Use a tube on the bleed valve into a container to save fluid pissing out on the bodywork or driveway. Bleeding afterwards is probably best to ensure a job well done but it's not necessary if you close the valve as you come to the end of pushing the fluid out. Fit pads and gently press the pedal a few times to replace the lost fluid and check reservoir. I don't recommend pushing the fluid back up the system. Not only are you forcing it through seals in the opposite direction they are designed to flow, you're also pushing the shitiest part of the old fluid (which is at the end of the line) back into the system.

you have no need what so ever to un-do the bleed nipples when just replacing brake pads, just un-do the resovoir cap and squeeze the pistons back, the fluid will flow back into it, when pads have been fitted just pump the brake pedal a few times for the pistons to come back out.

The system is designed to allow fluid to flow in both directions, otherwise the brakes would never release.

you're also pushing the shitiest part of the old fluid (which is

Why should the fluid in the wheel cylinders be any dirtier than the fluid in the rest of the system? Mike.

It is only *forced* under pressure one way.

Well, in my experience of bleeding brakes I find the first initial flow of fluid from the valve the dirtiest.

The message from "Mike G" contains these words:

In practice it is. The two places dirt can enter are the reservoir and the seals. This is a good opportunity to change the fluid that's got dirtiest and hottest.

That's debateable, but in any case, fluid is not being forced 'through seals in the opposite direction they are designed to flow' which is what you stated in your post. Fluid returning to the m/cyl reservoir bypasses the pressure seals in the m/cyl.

Possibly because it's picking up any crud in the bleed nipples or bleed tube. I don't think it's of any concern if the fluid is changed every 2 years as is recommended. Mike.