Loosing brake fluid no leaks?
Smoke from exhaust particularly on overrun?
This would normally be the seals inside the servo leaking, the fluid is then sucked into the engine via the vacuum servo hose and burnt.
Brakes and brake lights Sticking On?
This is when the servo vacuum chamber is full of brake fluid, the leather seal comes completely soaked with brake fluid, this can also cause the foam insert inside the leather seal to become enlarged. Misalignment of the cylinder can also cause sticking after repair.
With engine switched off, pump the brake pedal a few times pedal should go hard, then with foot still on the brake pedal start the engine, the pedal should go down further with same pedal pressure as the assistance works, if ok, next pump the pedal repeatedly the assistance should stay the same and the idle speed should increase slightly (only increases with old cars), if the assistance decreases there is a problem with the vacuum servo hose, these can calapse, make sure you use the correct servo hose when replacing, if ok switch off the engine wait five minutes then check that you still have assistance for one or more pumps, if the assistance disappears after 5 minutes you are loosing vacuum, this could be the one way valve or an air leak, to the test one way valve, repeat the test but clamp the vacuum servo hose after switching of the engine if assistance is still there the one way valve is leaking, if not you have an air leak in the servo unit.
I choose to have my cylinder sand blasted to give it an as new look, if you do this be sure you have removed all of the blasting dust before rebuilding.
Clean all parts thoroughly with brake fluid (I use silicon as it won’t harm paint work) fit new seals lubricating with the correct grease or brake fluid.
I left the leather seal rapped in a cloth for days it can hold a lot of brake fluid, a tip when overhauling a servo, is cut between 0.5-1.0 inch out of the foam insert (inside leather seal) and glue together, this will help with the sticking, don't cut to much or the seal between the leather seal and drum will be lost (some new kits have an excessively firm foam insert), be sure to thoroughly soak the leather seal in silicon lubricant not WD 40.