I agree with the test method mentioned by Ian.
The revs drop initially, but it stabilizes even with the brake pressed.
So maybe not this then? Could it be a carb issue - floats at wrong height? (think I read somewhere if you brake suddenly then revs can drop)
At sudden braking with maximum brake force, all atmospheric air from the servo drum
will be drawn into the engine. This leans the mixture for a short period of time.
On LHD 33s the vacuum for the servo is taken from the intake port of the 4th cylinder,
so the mixture balance of this cylinder will be affected. So we can say that, a short period
of irregular idle is normal when applying the brake suddenly.
If this drum has a small air leak is there anyway of testing this in the workshop? Not an easy thing to replace here (or cheap).
Yes, the diaphragm leak could be tested, even at home. You'll need:
- a vacuum meter (I guess you're already have one)
- a "T" fitting for the meter, which is suitable to insert into the brake servo line
- a valve (maybe a fish tank valve will be OK)
- a vacuum pump could be useful, but the engine vacuum could be used as well
To test both one-way valve and the diaphragm at once: insert the valve and the
"T" fitting (with the meter) between the engine (or vacuum pump) and the inlet of the one-way valve.
The valve should face to the vacuum source.
Start the engine (or the pump) and wait until the vacuum reaches -0.6 psi (or more,
up to -1 psi), then close the valve, and shut down the engine.
Check the vacuum drop on the meter. If the depression stays more than an hour,
then IMHO the valve and the diaphragm inside the servo drum are OK (try to avoid any
leaks, when the extra hoses and fittings are connected, it could give misleading results).