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I hope I'm not offending you by opposing what I believe to be a myth. But it upsets me to think how many "newbies" come across it, repeated endlessly here as "Gospel". My concerns about crushing the brake line may be unfounded, but in my opinion so is this brake line clamping tip. And to me it's not worth risking needless hose damage.
Yes, I thought the clamping idea probably came from the FAQ, but I don't consider this magazine tip to be a credible source—especially one with so many vague qualifying terms and a questionable understanding of the "path of least resistance" as applied to brake fluid. Here are my views on it:
"[Tip from Motor Magazine] Add the step of line-clamping the brake hose to the caliper when you retract caliper pistons. If you don't, you may be forcing brake fluid from the caliper bore (where rust and other debris may have collected) and sending this dirty fluid upstream. It's long been standard practice to open the caliper bleeder screws to give brake fluid a place to go while retracting pistons. However, sometimes the bleeders are partially blocked. In these cases, they may not let out all of the fluid that you meant to let escape. Instead, the fluid may take the path of least resistance - upstream. Nowadays, that's likely to send it to sensitive ABS components."
• This doesn't make any sense to me. Even a "partially blocked bleeder", would offer "the path of least resistance", when compared to the totally closed "upstream" path to the ABS modulator. Or does the writer suppose there is empty air space "upstream"?
IMO, this alone discredits the whole "tip", as presented.
"If the vehicle you're servicing has a blocked bleeder, you may get around the problem by cracking open the fitting where the brakeline attaches to the caliper before retracting the piston."
• With a "blocked bleeder", I wouldn't be "retracting the piston" I'd either be fixing the bleeder or replacing the caliper. Advice like this makes me question the writer's experience and credibility—and thus the whole "tip".
"For added assurance, you may also want to line-clamp the hose feeding the caliper with genuine clamps designed for this purpose."
• I've never seen any "genuine" brake hose clamps. Probably because I've never needed any. I doubt many DIY'ers have either. And I think it's wrong to send them looking for something that may not exist (or be worth using) based on a questionable magazine tip.
I do have line clamps that I use on pliable fuel, vacuum, or coolant hoses. But I don't agree with needlessly subjecting brake hoses (with their semi-rigid inner liner) to any crushing forces.
I don't know where you've "seen references" to clamping a brake hose to prevent fluid loss when changing the caliper, but it's just not necessary. Fluid loss from an open line can be easily prevented by blocking the brake pedal depressed (while bleeding off the resulting pressure). The MC piston seals will block off the reservoir ports so no fluid will flow, except for a drop or two when the line is first opened.
This method is in the 700/900 brake service manual, although not explicitly explained. (Maybe it's assumed to be common knowledge?)
If I knew of a convincing reason for brake hose clamping, I might try it (reluctantly). But this FAQ magazine tip doesn't convince me in the least. And I hope you will try the "pedal down" method the next time you have to open a brake line, instead of clamping it.
Bruce
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.
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