I've been busy with a disc brake upgrade om my ute and everything that could go wrong did, this simple converstion has become a absolute nightmare over the last couple of weeks. Without going into to much detail, here is my problem.

The discbrakes work great for about the first 10mins that im driving around but after that it slowly starts to build up pressure and starts braking all by its self. Ive narrowed it down to 2 things, 1 being the shaft on the booster is too long and consistantly applys pressure on the master cylinder or 2 my master cylinder is stuffed.

I think its the master cylinder cause when I blow on the hole for the rear brakes I can blow air through but when I try to blow air through the hole for the front brakes nothing. No air goes through.
Is it suppose to be like this or is this the root of my problem?
I asked the brake "experts" and got a reply "I don't know"