Coming home from BMCD in Roswell I experienced some unusual behavior from the brakes on the Stag in that the booster was failing and eventually left me with a very high pedal effort and weak braking. Now Stag boosters don’t grow on trees and an internet search turned up Power Brake Booster Exchange in Portland OR, web page www.boosterdeweyexchange.com A call to them revealed that they do know of Stag boosters having rebuilt one for one of the Stag prototype, all wheel drive cars that was fitted with a very early anti-lock braking system, Maxaret by Dunlop. I sent the booster to them and they returned it in short order. The brake master cylinder was leaking some brake fluid and this is what probably took out the booster so a new master cylinder was also sourced. I put it all back together, bled out the brake system and went for a test drive. What a difference, not so much in braking performance which is very adequate but in how the engine performs. The booster was obviously leaking vacuum for some time and excess air entering the engine via the booster leak was causing an erratic idle when sitting at traffic lights with my foot on the brake. It was also backfiring on downshifts. All that has gone away with the rebuilt booster and the engine will now start without any choke and when warm, idles at 650 RPM, probably 100 lower that specs.
There are several booster checks you can perform on any car that uses an inline booster system and also visually see if any brake fluid is leaking down the front of the booster. If your car exhibits any of the traits I mentioned above swap out that booster. Remember also that the youngest of our Triumphs is now 34 years old and the boosters were never built to last as long as many have.