My F6 Master & Slave cylinders are new, brake lines are new, the "Hydro-Vac" or vacuum brake booster is original. I have back pressure when operating the brakes such that the master cylinder vent cap leaks. Is this the vacuum brake booster malfunctioning? Where do you have it rebuilt?

 

It's not the booster. All the brake booster does is assist in pushing the brake pedal. If it is malfunctioning the symptoms are hard to push brake pedal and sometimes a vacuum leak. Sounds like the master cylinder is bad, letting fluid around the seals so that when the pedal is released it is pushing brake fluid
back into the reservoir.

 

It could also be that you don't have freeplay in the pushrod from the pedal to the master. There needs to be definite play there when released

 

The new cap on the left and old one on the right. The old one has a chamber that restricts brake fluid access while allowing venting.


Thank you TireDood and Ross for pulling me off blaming my vacuum brake booster.
I removed the after-market master cylinder cap and replaced it with the original master cylinder cap. The leaking has stopped!

 

Don't you love simple solutions?

 

Yes, and don't throw anything away until you're sure you're done.