Brake Troubleshoot - Novice

I mentioned in my first post in the welcome wagon forum that I am a novice mechanic. I have performed relatively simple tasks such as replacing alternators, a water pump and even replaced a clutch once on a GEO Tracker. With that background out of the way, here's my current issue.......

We purchased our mustang a little over a month ago in Texas and drove it back to Alabama. We have put approximately 2,000 miles on the vehicle. The last couple of times I have driven the vehicle, I have noticed the brakes getting weaker and more spongy. This last drive got my attention. When I got home, I checked everywhere I could for vacuum leaks. I could not find any. All hoses seem to be intact. I cut the engine and with the ignition on, held my foot on the brake. It sunk to the floor in approx 10 seconds. I pumped the brakes several times to build them back up and put my foot on the brake again. Same result. 10 seconds and the brake peddle was on the floor. When we first purchased, the brakes seemed to be pretty good. It seems the last 3-4 trips is when I started noticing it. The car was not driven much prior to our. purchase. We have probably put as many miles on it in the last six weeks as were put on it over the last two years.

Diagnosis?
Master cylinder? Booster?

Next question:
My goal is to build a nice street car. I would love to eventually produce equivalent of 400HP, but will start with the basics (brakes, body structure integrity, etc). I intend on building and eventually passing it down to my son when I'm old and decrepit. So, everything I do, I intend to upgrade.
The front has new drilled and slotted rotors. The rear has no upgrades.
If replacing MC and/ or booster what kind of upgrades are avaialble?

Sorry for the epistle. Just trying acquaint myself with the forum.

Thanks in advance.
-Kevin

I have been very consistent in checking all fluid levels since purchasing the car a few weeks ago. It also stays parked in a garage where I would notice leaks. To my knowledge it is not leaking any fluids. I have checked brake fluid level several times now. I have not had to add any. Level has stayed the same.

Mustang5L5 said:

I would try a brake bleed to rule it out.

Couple more questions. With the engine off, if you pump the pedal a few times, is it hard? Does it barely move, or will it slowly sink to the floor,

My suspicion is the master cylinder is failing internally and fluid is getting past the seal.

If the booster failed, your pedal would be rock hard no matter what. You'd have no assist.

Click to expand...

With engine off, pumped pedal several times, then held it down. Slowly sank to the floor in approx 10 seconds. Same thing it did without pumping the pedal. When the brake is released, it returns to correct position in less than 2 seconds.
Question about the brake bleed: if the brakes seemed to be functioning properly for the last 4 weeks, what would cause air to get into the lines? (If this is the issue)