[Mgs] Master cylinder - size of ports -1600 MKII

[Barney Gaylord]

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