Brake Master Cylinder, Brake Pressure Regulator?

Sorry if this questions been asked, i've had a browse through and done a search but cant find anything related.

I've just replaced the O/S front caliper and bled it, I had a good brake pedal for around 10 minutes and then it went to the floor all of a sudden. Thinking the brakes hadnt bled properly I tried it again, the same happened.

I'm pulling air into the system from somewhere and its definitely neither of the front flexi's, pipe's or calipers, nor is it peeing out of the drums or pipe work to the rear.

A friend told me that if anything pricey had gone pop under my bonnet, there would be evidence of corrosion caused by a leaky brake bit or signs of wet if the seals have gone, there isnt any of this.

Has anyone ever experienced similar problems and how did they determine what the problem was?

I'm no expert so detailed descriptions may be required (lol)

Thanks in advance

Matty

Hi Mattyclipper, I had a replaced all the brake system at the back and was bleeding all the system but doing this my master cylinder busted a seal (due to having rust or dirt stock to rod) which caused the brake pedal to go to floor from time to time. All was fine once replaced so my bet is on master cylinder.

Thanks for the quick reply!

I'll get one of those priced up then and hopefully be back on the road tomorrow!

Thanks again

Mat