Intermittent brake issue

[Benshae]

Hi All, my 2013 JK 3.6 Auto has an intermittent brake issue.
My driveway is steep, and when I reverse out, my brake will often grab initially, then when I release and re-apply, will grab with less force and then after another application, will proceed to the floor as I slowly continue backwards.
Touch-wood I haven't had any issue on the road as yet, only seems to be as I exit my driveway in the morning, with one exception:
The only time I have been off-road since this started happening about a month or so ago, took me about 30mins to drive to the track, then after around 15 mins of up and downs, I reversed on a steep hill to change my line, and the brake pedal went to the floor and I continued backwards... Pretty scary.

Few possibilities I've read could be air in ABS or faulty master cylinder.
There is no leaking fluid, vehicle has done 60,000 easy km's.

Any advice greatly appreciated. I think my next move is to have a garage bleed the ABS.

[Mudgee hunter]

Sounds like a vacuum issue perhaps. Inspect the vacuum lines to the booster.
It often can be heard leaking for a few seconds after turning the motor off. A whisper noise. Get someone to turn it off for you. And listen.

[Benshae]

Originally Posted by Mudgee hunter

Sounds like a vacuum issue perhaps. Inspect the vacuum lines to the booster.
It often can be heard leaking for a few seconds after turning the motor off. A whisper noise. Get someone to turn it off for you. And listen.

Thanks, I read that if were a vaccuum issue, the pedal would be hard rather than soft.. but I can definitely organise to test as suggested, thanks.

[Mudgee hunter]

For the small cost of a full flush of new brake fluid, its worth doing .
. There was a recent similar thread. Check to see if it is the same model as yours.
Master cylinders are usually made with crappy cast alloy.
If it is pitted etc in the bore.
Some engineering companies will fit a Stainless steel sleave.
It often works out about the same price.
But it will outlast a replacement in my books.
Pitted bores will .eek and chew out seals etc.

[Mudgee hunter]

Check the inside of the fire wall up near the brake push rod for a leak.

[Hayesio]

Sounds like your brakes just need a good bleed. If the fluid is more than 2 or 3 years old, just replace it. If the master cylinder was cactus your problem would be present all of the time and there would be a leak back into the brake booster and seeping down between the two.