I disagree. The reason to open the bleed nipples is to expel the displaced fluid and not force old fluid back into the system and particularly if you have an ABS pump. It also exercises the nipples so that you avoid seizures. Regards Peter

Ta for the replies.

Just for clarification, the Haynes manual does advise bleeding the system via the caliper nipple when changing pads.

I haven't changed that many sets of pads/discs over the years--perhaps a dozen or so altogether--but on every previous occasion I've released the pressure by taking the cap off the master cylinder reservoir. The Haynes specifically warns against this method due to the danger of reversing seals.

Am retired now and in the 27 years I had my garage business

I never heard of or had master cylinder seals reversing.

I would say if the seals were soft enough to reverse the master cylinder was due to be replaced.

It is a good time to change the brake fluid at the same time as the brake pads.

Brake fluid is hydroscopic (absorbs moisture from the atmosphere).

If you use the bleed nipple you are likely to pull the moisture contaminated fluid from the master cylinder reservoir, as the reservoir is open to the atmosphere.

I always asked the customer if I would change the brake fluid at the same time as changing the pads, if they did not want this I would just push the Caliper pistons back using a G clamp.

If you have damaged the bleed screw threads you will feel the pressure easing off as you finally tighten the screw.

Edited by scotmech5 on 31/03/2013 at 13:30