Ebrake does work, hydrolics do not

91 240sx.

So the other day I was driving to work and my car started to slow down while having plenty of throttle.. i thought maybe my clutch was slipping.. so I press in the clutch and the car started to slow down hard core.. I press the brakes to pull off to the side, and my brake pedal is all the way out and stiff as a mofo.

I finally pull over into a parking lot(pressing brake really hard) and when i stopped - smoke starts coming off my front wheels like I did a fwd burnout. Both front brakes were close to being on fire it seemed.

I couldn't figure it out at the time, i called a friend and got a ride to work.

I came back after work (about 9 hours) and was about to call AAA, but I wanted to try it again, and my brakes were fine..wtf.. pedal was soft enough, not stiff at all.

I check the brake booster, vacuum was on fine.. brake fluid fine..

Someone told me it may be the brake booster going bad (not the calipers, because both locked up at the same time), luckily I have 2 240s.. so my other car became the donor.

I swapped the boosters, and used a brake line air hose vacuum thing to bleed the brakes, fluid came out of the fronts, but not the rears.

Finish up the booster, and the bleeding, new fresh clean fluid flowing through the lines, master cylinder full.. press the brake, and I have no rear brakes.
Pull the ebrake, works like a charm - locks up both - any ideas??

Sorry this was long winded.. just wanted to throw out all that happened.

Think it is the master cylinder??