At 08:51 AM 10/30/2008 -0500, Pat Harris wrote:
>....
>"Blocked port" aspect. When the piston in the master cylinder is in
>its correct "at rest position" is the only pathway for fluid to
>return to the master cylinder via this small port?
Yes.
>If so should pushing the caliper piston back into the caliper cause
>the brake fluid to move into the master cylinder via this pathway?
Yes.
>If so, would this then be an indication that the small port was not blocked?
>....
Yes.
However, if the master cylinder piston was sticking in the bore and
not returning all the way to rest position the little hole would
still be blocked which could cause brake drag. Pushing a slave
piston back in the bore may produce enough pressure in the line to
pop the master cylinder piston back to the rest position giving the
impression that all is well when it isn't.
You still need to determine where the fluid return restriction is
happening. If you crack the line fitting at the master cylinder and
the pressure is relieved, then you know the problem is inside the
master cylinder.
When the master piston is returning properly to rest position, but
you still have restriction to return flow, then check the slow return
restrictor valve in the brake master cylinder bore. This could be
malfunctioning to stop back flow at low pressure. This is a check
valve with a slow return bleed orifice. It should allow free flow of
fluid in the forward direction for quick application of brakes, and
give slow return of fluid in the reverse direction (via a small open
orifice) so you can "pump up" the brakes if you have long pedal
travel. The small open orifice is supposed to allow fluid to slowly
return dropping to zero pressure in the line.
If the small return orifice is clogged it can give the symptoms you
describe, allowing some fluid to return at moderate pressure (by
upsetting the entire valve assembly from the seat in bottom of bore)
but holding a small amount of residual pressure in the line (due to
spring force against the check valve assembly). In other words, if
the return orifice is clogged it will act like a pressure relief
valve to maintain a certain amount of pressure in the line (similar
to the pressure relief valve in the engine oil system).
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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