What is the right way to bleed off excess hydraulic fluid?

Dear Car Talk

Dear Car Talk | Sep 01, 2000

Dear Tom and Ray:

I need to do a brake job on my 1993 Mercury Grand Marquis, and I want to know the correct procedure for bleeding off the excess hydraulic fluid. Some mechanics have said that when you depress the caliper piston to make room for the brake pad, the excess fluid should be vented there at the bleeder valve near the wheel and then discarded. Others have said to just push in the piston, forget about opening the bleeder and let the extra fluid go back up the lines to the brake master cylinder. Which procedure is correct, or doesn't it make a difference? -- Doug

RAY: Well, in the old days, you used to just push in the pistons, and the stuff would go back up the lines and end up in the master cylinder. Or if the master cylinder was full, all over your garage floor.

TOM: But now, with anti-lock brake systems (ABS) so common, you're no longer supposed to do that. We've found that when we force fluid -- and more importantly, other debris -- back up the brake lines, it can cause damage to the ABS.

RAY: So the preferred procedure is to bleed the excess fluid out right there at the caliper. Just grab a container, open the bleeder, push in the caliper piston and catch the overflow. It's no more difficult to do it that way, and it beats having to spend $2,000 on a new ABS unit, doesn't it?

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