Air In Brake Line 2007 YZ450

There isn't an adjustment at the rear master cylinder that the vertical position of the operating rod and the piston (which makes it different from the front). The return spring runs the rod assembly back to the bottom, where it stops against a circlip. The adjuster screw changes only the at rest position of the brake pedal, and since the pedal has no fixed stop other than the M/C itself, the plunger position does not change.

None of this is true.

The caliper piston does not return anywhere, in the first place. It is advanced only marginally, and if it retrearts from the rortor at all, it is only due to the runout of the rotor, and the play in the wheel bearings. Having it stand more than a very few thousandths away from the rotor creates excessive pedal/lever travel when the brakes are applied, and the only thing necessary to affect a complete release of the brake is for the pressure to be let off the pads.

There should also never be a vacuum in the system either, and the cap on either master cylinder is vented to prevent it.

When you release the brake, the lips of the master cylinder piston seal(s) fold down, allowing fluid to flow down over them from behind (where the reservoir is), so they don't apply any vacuum that way, either.

When i said the piston returned i didn't mean a 1/2" away from the rotor. i was talking like .005" actually

Also you say there is absolutely no low pressure (vacuum) in the system. Then how when you release the master cylinder and the cup seals fold down... what causes this.. hrmm maybe a pressure difference on each side of the cup seal.. define pressure difference . higher pressure on one side lower on the other side.nice ... lastly Wow you have worked on corvettes.. so have I..

When i said the piston returned i didn't mean a 1/2" away from the rotor. i was talking like .005" actually

Also you say there is absolutely no low pressure (vacuum) in the system. Then how when you release the master cylinder and the cup seals fold down... what causes this.. hrmm maybe a pressure difference on each side of the cup seal.. define pressure difference . higher pressure on one side lower on the other side.nice ... lastly Wow you have worked on corvettes.. so have I..

Then you should be familiar with the fact that the 4 piston rear calipers on the C3's were notorious for pumping themselves full of air when the rotors warped because of the pistons passing fluid back and forth across the caliper.

The pressure differential you mention is transient, and is gone after the piston returns, due precisely to the fact that the piston seals allow it to be normalized. Given the ease at which the master cylinder returns, the very low strength of the pressure differential, and the force required to move the piston in either direction, there is simply no way for such a brief, weak vacuum to pull the piston back at all.

Any retraction of the caliper piston from its most extended position is only due to rotor runout, bearing play, caliper mounts out of square etc. No design element of a hydraulic disc system deliberately retracts the caliper piston or pads.

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