hey all, my brakes haver recently started to lose pressure at the most random times. ever since i changed the diff in the car and bled all the lines this problem has started. the first time, luckily just as i was coming up my driveway the brake pedal went all the way to the floor and the brake warning light came on. so i bled the lines again thought nothing of it(after the first time i did it my mate forgot to put the lid on the booster amd closed the bonnet so i drove with it like that for 2 days:bang then it was all fine for a week until i get up to go to work in the morning and get to the end of my street to find the same problem so i left it at the mechanics for the day and they told me i need a new master cylinder. so i pick one up on the way home put it all on bleed the brake lines again, after bleeding the lines the brakes felt really stiff and had a nice feel but after 3 days they feel spongy again. is this what should happen or could there be another leak in the lines?