External master cylinder/vacuum booster leak
 

I have a leak between the master cylinder and vacuum booster. The leak appears to be where the master cylinder connects to the booster. I had leaking at the top of the master cylinder, but I replaced the seal under the master cylinder lid and the leaking stopped from that area. I have had master cylinders leak internally, but never externally. I sugged up the two nuts attaching the master cylinder but it did not stop the leak.

Is there a fix other than replacing both the master cylinder and the booster? These are AC Delco units, which only have about 30K miles. The brake fluid level is OK and the brakes work fine, and the brake light is not lit on the dash.

Re: External master cylinder/vacuum booster leak
 

The seal is leaking at the rear of the master cylinder. It needs replaced. If brake fluid got inside the booster, the diaphragms days are numbered.

Re: External master cylinder/vacuum booster leak
 

Most likely the seal around the master cyl input ...

Re: External master cylinder/vacuum booster leak
 

I see I have a choice of JB6 or JB7 brake master cylinder and booster. Since I have an 8600 GVWR, can I assume that I need JB7 parts? I have no SPID on this truck. I have a 14 bolt full floater rear.

Re: External master cylinder/vacuum booster leak
 

Based on my past experience with this truck, I can replace the master cylinder and see if the booster still works properly. If it fails in the future I can replace the booster later without compromising the master cylinder, and without needing to reopen the hydraulic system - correct?

Re: External master cylinder/vacuum booster leak
 

You should have JB7 brakes. You can replace the booster later without opening the system.