1974 tii Master Cylinder doohickey question

[0257]

Noticing a bit of bubbling paint on the frame rail under the master cylinder, I observe that the rear brake pipe is a bit rusty/moist where it exits the master.  Also there’s a bit of rust above the doohickey (some kind of adapter/limiter?) into which the brake line is fitted.  I think there must be a slow leak at this juncture, either above or below the doohickey.  Can anyone tell me what that doohickey is, why it’s needed, and — bonuse points — what is likely causing the leak and whether replacing the line is likely to be a fix? Thanks.

[Son of Marty]

That's your rear brake pressure residual valve, it keeps a small amount of pressure on the rear brake cylinders to keep the shoes close to the drums, it reduces pedal travel by quite a bit. First step I'd try is replacing the crush washer see if the leak stops.

[jimk]

Run your finger under the MC at the bolting flange to the booster.  If it shows brake fluid, its coming out of the seal weep hole and the seal is failing in the MC.  If so and the MC is not replaced, fluid will eventually be pushed into the booster.

[0257]

1 hour ago, jimk said:

Run your finger under the MC at the bolting flange to the booster.  If it shows brake fluid, its coming out of the seal weep hole and the seal is failing in the MC.  If so and the MC is not replaced, fluid will eventually be pushed into the booster.

[jimk]

That's not the weep hole.

[stephers]

No it is not but it is a doohickey

[jimk]

5 minutes ago, stephers said:

No it is not but it is a doohickey

Why quote the msg on the weep hole and show a doohickey?

A simple search shows the weep hole is here:

 

[0257]

Happily, the juncture at the booster is dry and dusty.  Thanks for the tip.

 

Whoops, my photos from that last post ended up as a response to S. Marty’s.  For anyone who’s never seen a residual pressure valve up close.?