Brake issues

[1fast4]

Ok, before I go throw a bunch of money on parts here is the low down:
88 F150 I picked up from an auction. No idea how long it had been sitting around not driven or moved.

Started with pedal that would push to floor.
Bled brakes - (farthest to closest) a few times and pedal nice and firm
Start driving around and slowly pedal starts going soft again.
Bleed brakes again. Same issue
Took it somewhere and paid to have it bled and same issue.

Now, when I push on brake pedal quickly and hold it, the pressure stays.
If I push slowly like coming to a stop, pedal pressure fades.
If I give it throttle, brake pedal comes back up in its own.

The vacume check valve to the booster is working as it should.
VERY good vacume to the booster.
NO leaks of brake fluid visible anywhere.

Could the booster have a leak?
Master cyl bad?

Thoughts?

Thanks in advance. I would really like to get this one fixed so I can start driving it around.

[soap]

i've had that same problem on my 90 GT. it was the master,

[Blackpony]

Master cylinder is bad

[SC457A]

Master cylinder is bad

Yep, buy some parts

It is bypassing internally.

[dee]

Yep, buy some parts

It is bypassing internally.

X2

[1fast4]

Thanks folks.
I'm not opposed to buying parts, especially for a safety related item, I was just wanting to avoid throwing money at parts!!

Thanks for any and all I put.
Will be getting a new MC next.

[garycrist]

I would agree providing, there are no visible leaks at any wheel or hose.

[1fast4]

Thanks for the input folks.

Replaced the MC and issue

Project cheap ass is now on the road!!