Took my mini out for a quick run this arvo ( disc brakes , VH44 booster ) i jumped back into it after going to the bank , went to pull up at an intersection and the brake pedal felt really hard all of a sudden , did a couple of brake tests in it , yep pedal pressure was way harder than normal , first thought brake booster has sh!t itself , took it home , checked for obvious issues everything fine looking at it.

I noticed that when i put my foot on the brake pedal when idling the engine revs increase , i'm thinking diaphragm knackered , any thoughts?

Check the little elbow hose Under the cylinder on the booster. I don't know what it is called. it is only about 3/8 in diameter. it could be cracked or split and sucking air.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
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With the engine off, put your foot on the brake pedal, now start the car. Does the pedal go down a little?

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

Finally got 5 mins to have a look around the booster today , i can't see anything out of the ordinary with the small elbow hose , nor the hose that runs back to the inlet manifold .

It has a high brake pedal without engine running , start engine , no decrease in pedal height , which leads me to think that something is going on internally with the booster , just no idea if it's in the
diaphragm or the m/cyl part .

With engine running , depress brake pedal ,engine revs increase by a couple of hundred rpm's .

Sounds like diaphragm has a hole. Kit time...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.