Good brakes....usually.

[rob.hardy1]

Took my traveller out for a run today and had a brake problem. Pedal is good .....mostly.Just occasionally the pedal would go almost to the floor and brakes they no work much.One quick pump and back to normal. No sign of fluid leakage and the fluid level still good.
Any ideas where to start

[bigginger]

Bleeding - and checking again for signs of a leak
a

[ian-s]

If there's no leaks at the wheel cylinders , it can only be the master cylinder , the rubbers have probably lost their shape and only seal by virtue of the pressure on them. change it before you hit someone/thing next time the pedal goes to the floor

[Orkney]

Fingers crossed it might just be a dastardly airlock had the same before i renewed everything, but always from cold, not whilst driving.
- Like BG said bleed them first, get it on good & level hard ground, jack the back up both sides.
Make sure you dont have any upside down 'U' bends around the diff or T there in the brake pipes - if you gently get them bent flat so when you start all the pipes are facing uphill to the back.
That way you know for sure your expelling all the air.
Takes very little air in a trap to cause this, picture syponing liquid using a clear tube and theres an air bubble at the top of the highest point. Sometimes some of it will get sucked downhil with the flow as small bubbles, soon as you stop syphoning they come right back to where they were.

[DAVIDMCCULLOUGH]

Has the car stood for a long time and then been put back on the road? I would agree with Ian s that its probably the master cylinder on the way out. The old rubbers inside can start to brake up after a while and not do there job.