Problem with brakes

Hi All. Just bought a 1993 Hiace Super Custom converted to a camper van. Loving it. A Japanese import so I'm nervous about spares and does anyone know if a workshop manual is available?
Have a worrying problem with the brakes. Every now and then the pedal goes flat to the floor. It sorts itself out and is not losing fluid. Anyone got any ideas?

Worn mater cylinder, worn servo or sticking brake - all of which need looking at even though the bus is stopping ok!
Maybe worth a bleed through and see what new fluid does first?

Hi Clive

Yep, the master cylinder seems to be top of the list of possible causes. It is at a garage at present which clams to be able to get parts and know the vehicle. I warned them about needing to remove the dash and they said they knew and were relaxed. I see you are Bristol based, this garage is in Hengrove. I'll let you know how I get on.
I did replace the fluid and bleed the brakes. Good for 1000mls then the problem returned.

Parts easy to get, getting to it is the problem, Hope thw bill is small

Total of £350 plus VAT for part and fitting. Brakes feel OK, I'm off to Cornwall so that will be a good test. They seemed relaxed about removing the dash. I did have to take out the radio and reconnect the aerial but that was simple.

Well if you come back on brakes solved!

I'm back. The brakes feel fine. Off to Ireland later in the month so an even bigger test.
I do have a problem that I will probably do a separate post for. The conversion has a three person bench seat that can face either forward or back and has a hinge mechanism that allows it to fold flat to be part of the bed. This mechanism has broke so I'm looking for new mechanism or seats from a scrap vehicle. No luck so far.