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Normal for a few strokes as you advance the pistons to move the pads to the rotors. You need to avoid stroking to the floor as you do that as you can damage the master cylinder (MC) seals otherwise.
If you have done what it take to advance the pistons and the problem is persistant, then the MC is damaged. The brake lines do not need to be opened to replace pads and rotors. So there is no way to get air into the system so no need to bleed the system. Too much fluid in the reservoir will not create the problem described.
Description really sounds like bad MC. The lower half of the petal movememt is not normally used and this is where rust deposits form. When you full stroke the petal, the the soft MC cup seals get cut on those doposits. One the problem starts to occur, the seals get damaged all the more. So follow advice about never full stroking a MC, unless new. And to avoid this problem, bleed the brake fluid to prevent rust in the MC. I do not bleed every year, and the only place for moisture to enter the system is at the vented MC. So I remove all of the fluid from the reservoir and fill with new every year. This removes most of the moisture from the system. This should be done prior to a bleed job to prevent flushing the most comtaminated fluid down through the system. To do a really good job, the pistons should be pushed in to push as much fluid back to the reservoir before R&R fuild there. And one step further is to pump new fluid down into the calipers, with pads/spaces/rotors in there to prevent blowing the pistons out, then opening the bleeders and pushing in the pistons, otherwise the fluid trapped in the calipers is not well flushed.
To replace the MC, connect the lines loosely to the new or reconditioned MC, then fill reservoir with fluid and let it dribble out of the loose lines into a nest of rags. Then tighen up the pipes to the MC. Now push pistons at calipers in to back flush to get air out of the works. This avoids getting air into the system, and you definately never want to get any air into the ABS, as that can be difficult to bleed effectively (so I have heard). After that is done, one can do bleeding from there to renew fluid in the system.
These brake systems with vented reservoirs are totally insane. It is possible to use a diaphram to separate the fluid from the atmosphere, and detroit vehicles were doing this before many TSN members were born. So this situation is stupid and preventable!
posted by 68.95.11...
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