you can try a vaccum pump brake bleeder.
basically what i see happening could be 2 things.
1) defect in a part of the brake system
2) you have lots of air still in the system
if the situation is #2 what is happening is you have micro bubbles of air in the fluid. you bleed it and have a solid pedal, leave for 10 hours
next day you come back and the pedal goes to the floor, because all the micro bubbles have condensed back into large bubbles in the brake lines.
if that is the case then the question is how are you bleeding the system.
you should have a fair amount of hose coming off the bleeder going into a cup of brake fluid. so once air i bleed from the system it cannot go back in.
then you just crack the bleeder a tiny bit, because if you open it half a turn to start the fluid going, air will leak back through the threads on the bleeder. you just want it open enough to release the fluid and get air out not have it freeflow out the bleeder.
next you want to make sure you have the brake bleeding in the correct order. longest line first, shortest line last.
Passenger rear, driver rear, passenger front, driver front is the order from longest to shortest.
the amount of fluid you use, don't be shy when i flush the system out or re-bleed i may go through a small bottle just to make sure i get all the junk out of the system. basically install the hose on the bleeder rear passenger, but the hose in a cup of brake fluid, open the bleeder a crack, and pump the fluid in the master down to near the bottom, then refill. and pump again. if you use a clear hose on the bleeder into the cup you will see tons of air bubbles.
once you have pumped the master down 2 or 3 times, close off that bleeder and move to the drivers side and repeat, getting all the air out, and you may pump down the master 2-3 times. yup its going to waste fluid, who cares just get the air out.
then move to the front, passenger and front driver remember to just crack the bleeder to get the brake pedal to pump the fluid do not open it more then 1/8 a turn. that will keep air from getting back into the system from the threads of the bleeder.some people pack grease around the base of the bleeder to keep air out then clean it off later.
now if you did that for an hour or 2 then the next morning came back and the pedal went to the floor again, i would start to suspect the master then prop valve and i would want to inspect all the brake lines under the car at the joints for wet spots or a bad crimp on the rubber lines for the brakes somehow letting air back into the system. because if i keeps doing that there is a leak somewhere pulling air back in.
there could be a problem with the master or the prop valve. the master could be letting air in through the piston going to the booster. problem is you need to pull the master out to look back there and see if it is wet. there is a weep hole on the bottom of the flange that connects the booster to the master. do you find brake fluid has weeped down the front face of the booster. might indicate a piston leak on the master, or you could have a leak from the master cover.
a bad prop valve is hard to determine, you would want to have a spare to swap in then if the problem stops you know the prop valve is bad. those can develop an internal leak between the drum and disc sections sometimes you see it leaking from the plastic sensor on the prop valve that tells the system if it is leaking on either the drum or disc side of the system.
I assume your brake warning light is working inside the dash and isn't lighting up all the time.
usually it lights up when you start the car then turns off when the engine is running.
i know one guy had a problem and his brake warning light on the dash was burned out and he didn't know but his pedal didn't feel right.
for right now i say go back to pumping air out of the brake lines and lets see what happens.