Brake question, pads drag slightly

After rebuilding the single front disc brake system, both master cylinder and caliper, on my '77 650B, I have noticed the pads never totally leave contact with the rotor. If the front end is raised to let the wheel spin freely, the brakes show very minimal resistance to free spinning. The pads are new and thick as they'll ever be. I was thinking that the spring in the master cylinder assembly ought to pull enough fluid back and retract the pads so they don't touch the rotor. Correct me if I'm wrong here, but maybe I don't quite have every bit of air out of the system. Would this cause the pads not to retract fully? The brake handle doesn't feel spongy, although it does pull back 3/4ths of the way to the grip. There have been no leaks detected in the system.

You didn't say how "minimal" that drag was, nor how many miles you have on the pads since te rebuild...

Especially after a fresh rebuild, a "minimal" amount of drag is fine, as long as it doesn't significantly impede rotation of the wheel.

Your disc (you didn't say if it had been turned, so I'm assuming not) has a given contour, based upon wear, that gives it an irregular surface - and a small of that is not a bad thing. With some miles, the pads will wear to match that pattern - grooves and valleys - and work fine with significantly reduced resistance.

IDIOTIC ANSWER ALERT!

The lever slop isn't quite right. Might be cable a adjustment issue; there should be about 1/8" to 1/4" (max) travel on the lever before it starts to take hold.

Check the cable adjusters and try bleeding the brakes again - easy piece of mind for 25 cents worth of fluid.

Post edited by: Pterosaur, at: 2007/04/06 22:28

Thanks, Pterosaur. It is very minimal drag, zero miles (just blocks to test brakes, working on a terrible carb problem). I ran a pint of fluid through the lines. All parts are new. New lines, too. Maybe just a little more bleeding, I guess. New pads adjusting themselves to irregular rotors (fairly smooth, but not re-turned) sounds like that is it. I'll check as I start to rack up the miles! Cheers.

cable?what cable?:blink: :S :dry: :huh: :ohmy: i aint one of those engineering types but im thinking that cable you refer to we will call ex-ib-it A is actually a hydraulic brake line and the closest thing youre gonna get to adjusting said ex-ib-it is to bleed the brakes

wireman wrote:

cable?what cable?:blink: :S :dry: :huh: :ohmy: i aint one of those engineering types but im thinking that cable you refer to we will call ex-ib-it A is actually a hydraulic brake line and the closest thing youre gonna get to adjusting said ex-ib-it is to bleed the brakes

Ooopsies! Senior Moment! :blush:

I dunno why, but I was thinking about the clutch cable adjuster - pants - ankles - down - around. :whistle:

Hey! Give an old fart a *BRAKE*, will'ya! I got that pad part right at least!

:ohmy:

:dry: :pinch: :blush:

WirdOOd sez: "When only the *reddest* face will do, make sure it's Rustoleum (tm) Farm Equipment Red!"

Post edited by: Pterosaur, at: 2007/04/06 22:18

Disk brake pads always have a slight amount of drag, that spring is there to push the cup back to allow more fluid to enter the chamber and compensate for brake wear (as well as allowing brake bleeding).

If that spring was there to pull the pads back, what would compensate for pad wear? The distance between the pads and the rotors would increase as the pads wore, up to the point where the brakes ceased to function.

There is much sense in your words, master Steele. Of course! More fluid is drawn down out of the resevoir to fill added space in the chamber forcing the piston out; as pads wear progressively, the fluid level drops requireing regular level check. So when the brake lever is relaxed, the very high pressure in the lines and behind the piston then relaxes and the whole system goes limp. (sound like my wife speaking) This is where and why disc brake pads always have a slight amount of drag. This also explains why the lever shouldn't have to travel very far to firm up if the system is bled out correctly. I'm just not getting all the damn air out, that's all. Thanks for the illuminating thoughts. Back to bleeding brakes, no pun intended.