What's this hole in master cylinder?
 

I'm upgrading my 68 gmc to power disc brakes. This hole in the front of the master cylinder is leaking fluid. I don't know what it's for. Should it be plugged? Thanks.

Re: What's this hole in master cylinder?
 

the hole pointed to with the green arrow? If thats the "hole" then no it should not be leaking from that hole. manufacturing defect

Re: What's this hole in master cylinder?
 

Yes, that's the one.

Re: What's this hole in master cylinder?
 

that is a center drill hole. the factory uses that to support the rear of the master cylinder then they were turning the round portion (opposite end) on a lathe. can't see how it would leak fluid as it does should NOT go thru the casting.

Re: What's this hole in master cylinder?
 

Yes is just a center hole, I worked in engineering in a factory that made brake parts.

If it is REALLY leaking out that you need to replace that master NOW! Do not wait. If fluid is leaking out it coming straight out of the bore of the cylinder. Casting is probably cracked.