Mur. You're not 'calling me out' this time?

If you look at how a booster works, nothing goes on till the brakes are applied. Recently someone wrote that if a remote booster was leaking it would rev up all of the time. That is not the case. The valve in line to the booster is to protect it from a backfire that would pressurize the intake manifold. Early cars had this on the intake manifold and later cars had it in line. In any case, disconnect the vacuum line and plug it to troubleshoot the issue, and then replace the servo or remove it, but do not drive the car with it disconnected. Usually a seal failure in the booster is present when a valve or diaphragm issue is present.

IMHO it has to be on the vacuum side of the booster..  The liquid and air sides of the booster are totally separated by the large diaphragm.  If this is incorrectly fitted or damaged then you will get a vac leak which can cause either an air leak at the carb (raising the idle) or, in more extreme circumsatnces, brake fluid to be sucked into the intake ..which causes the white smoke.  Its possible that the air leak is in the vacuum line somewhere or its connectors????

While its a personal choice, I much preferr boosted brakes.  The whole reason its there was to allow the use of harder pads that would not fade as quickly under competiton use.... Yes you can just stomp harder ....but having had a diaphragm burst halfway through a hillclimb weekend I was gald to get it fixed...

Noting that you are using non standard masters anyway it would be possible (maybe more effective) to use correctly sized cylinders and dispense with the booster.

Cheers, Ian 

Michael:

I had this situation on my MKII.

It was strange, but everything worked correctly.

About 30K miles later, I started to see white smoke from the exhaust pipe when I bled the brakes.

I bled the brakes with the engine running.

Brake fluid in the intake.

I took out the servo.

That fixed the white smoke problem and the strange rev-up went away.

I also built up the muscle in my right leg a little.

The power brake looks nice on an authentic early Cooper S or 1275 GT, but it did not seem to help stopping much.

Disconnect the vacuum hose at the intake manifold, plug the hole in the manifold as Doug suggests.

You will not have power brakes, but your car will stop ok.

Russ

The late model Mini servos have a valve in the vacuum line i don't see one on your set up (maybe you don't need it) just a thought. It seems normal when you press the brake pedal several times on a servoe'd Mini the idle will rise slightly but not as much as yours. As others have said disconnect it do some more "heat cycling" and see what it does, don't worry the brakes will still work adequately for testing.

Not sure from the pictures exactly what you have but I guess a direct acting servo because the problem only happens applying the brakes. If a remote servo was leaking the it would rev up all the time. Do you hear an air sucking sound when you apply the brakes? The leak is probably at the seal around the servo push rod.

Servo was the first thing that came to mind.

You may still feel the servo working (assisting), but a vacuum leak will cause a raised idle. Problem usually is not the large diaphragm, but in the valve block.  

I'm sitting on several boosters and some rebuild parts, I've wondered if it would be worth it to install the booster because of problems like this.

On the fence on this one, hope I don't end up like zippy's squirrel. 

As Mur said, disconnnect the servo connection to the intake manifold and install a plug in its place.  Then test drive to see if the situation improves.  

Brake pedal. Disconnect the servo and plug the manifold, then apply the brake. It does sound like a servo issue.

Image Gallery

I’ve been doing laps around the neighborhood wringing out the last of the bugs on my boot to bonnet (thanks Nick!) refurbishment. I tell the wife I'm heat cycling “Rosebud’s” new motor. I’ve solved most of the issues associated with a new rebuild, but here’s one thats got me stumped.

Whenever I apply pressure to the brake pedal the engine revs pick up about 250 RPM. I’ve checked the peddle box and bulkhead for interference and any signs of flex. I’ve followed the peddle linkage up to the master cylinder and checked all the fittings on the hoses to the booster. Choke linkage checks out.

Having struck out on the obvious mechanical things, I’m beginning again to suspect a vacuum leak. Here’s a couple of shots of the booster and other vacuum thingies. The booster is tucked up under the right wing (thanks Mike!) so that's the best I.D. shot I can get. I’d appreciate any thoughts on the matter.

I’ve been a regular MM forum reader since I purchased my Mini 7 years ago. Between work and Rosebud spending several years in and out of various shops, it was only recently that I’ve been able to get my hands dirty on her. I have always enjoyed the info presented here, and now that I have some questions of my own, I suspect I’ll be logging on regularly for the next few months (years?).  Thanks!