On 2007-01-23 17:31, 66GT289 wrote:
On 2007-01-23 04:17, dennis111 wrote:
Drum brakes have a little umbrella type residual valve found underneath the brass seat were the brake tubing is attached. Spring pressure from the brakes overcome this valve and allow fluid to return to the MC when the pedal is released. You can see this seat when you remove the seat using a screw/ bolt of the appropriate size. This valve is included in MC rebuild kits for drum brakes and there will be 1 under each seat that is served by drum brakes.
Disc brakes do not use this valve as there is no inherent pressure to return the fluid to the MC when the pedal is released.
Might be splitting hairs, but....
Residual valve in a drum m/c is there to keep some fluid
pressure in the lines. You don't want much fluid bleeding
back into the m/c. The cup seals in the wheel cylinders
being solidly up against the wheel cylinder walls is the main
reason and yet another usually overlooked concern is
that if a bunch of fluid bleeds back, the shoes will be too
far away from the drum. If this happens, you will have some
braking "issues" - that I will guarantee. Diagnostically, it
feels similiar to a bypassing master cylinder that must be
"pumped up" to get a pedal.
Disc caliper system doesn't have these concerns, you can
let all the pressure bleed back to the m/c. On some GM
products they use a special caliper seal and m/c to improve
fluid return to the m/c after braking- that way the caliper
piston will retract even more, the pad won't be dragging
against the rotor as much and fuel economy is improved.