To bleed or not to bleed?

[RR76]

Do you need to open the bleed valve when removing or replacing the brake caliper?

[sbk1972]

Are you disconnecting the caliper from the car / brake line i.e. replacing the caliper ? If so then yes. Anytime where air can enter the brake system then it will need bleeding afterwards.

However, If you are removing the caliper, pushing it to one side to replace the brake pads, then no you shouldnt need too.

[RR76]

sbk1972 said:

Are you disconnecting the caliper from the car / brake line i.e. replacing the caliper ? If so then yes. Anytime where air can enter the brake system then it will need bleeding afterwards.

However, If you are removing the caliper, pushing it to one side to replace the brake pads, then no you shouldnt need too.

I'm replacing disc and pads not removing brake line however many videos and guides say you should open bleed valve?

[steveo3002]

ive never needed to as a home diy'er ....see if the piston will push back , if it wont then yes might have to crack the nipple

[InitialDave]

You shouldn't need to, pushing the piston in will just send fluid back to the master cylinder reservoir.

But cracking the bleed topple while pushing the piston in will squirt it out there instead.

Tbh I only do it like that if it looks like someone has topped up the fluid such that it'll overflow the reservoir if doing it the first way, or if I'm going to be bleeding through anyway.

But I do prefer to check all the nipples are moveable if working on the brakes, even if the current job won't involve bleeding.

[AllyBassman]

A less messy approach would be to check the level on the reserviour, and if it's high, remove some fluid with a syrine or pipette.

[InitialDave]

Using the bleed nipple isn't messy, it just goes down a bit of tube into a plastic bottle or whatever.

[Spurry]

Just put a clamp on the flexi pipe, open the bleed valve, then push pistons back, and close bleed valve, remove clamp. Easy.
This method ensures that any possibly contaminated fluid in the lines is not pushed back into the master cylinder.
Has the added advantage that the master cylinder will not overflow.

[GreenV8S]

No. It's a recipe for getting air into the lines while you're dealing with the caliper. Keep it sealed until you come to bleed them.

Unless somebody has been topping your brake fluid reservoir up behind your back there's no danger of overflowing the master cylinder as long as you pump the pedal after you finish working on each corner to reseat the pads. And if somebody *has* been topping up your brake fluid then that's a serious problem and you need to stop them from doing that.

[Captain Answer]

Opening up the brake fluid resevoir cap helps with when you wind the piston back in, don't usually touch the bleed valve unles replacing pipe/line or brake fluid

[SiH]

Apparently some ABS systems can be a bit sensitive about pistons being pushed back and causing brake fluid to flow the 'wrong way' in the caliper. It's thought that by opening the bleed screw you can push the piston in and the fluid comes out of the screw rather than heading back up towards the master cylinder. I'm not sure if this is a real thing or some hypothesis that has become received wisdom.

[Order66]

SiH said:

Apparently some ABS systems can be a bit sensitive about pistons being pushed back and causing brake fluid to flow the 'wrong way' in the caliper. It's thought that by opening the bleed screw you can push the piston in and the fluid comes out of the screw rather than heading back up towards the master cylinder. I'm not sure if this is a real thing or some hypothesis that has become received wisdom.

This is correct. On many cars you're better to open the bleed valve. Gives you a mini-bleed, stops the valve seizing and keeps the abs happy. Just make sure the M/C doesn't run low.