Sinking brake pedal

So I got this 620 (GSi, 2.0 Honda) off eBay, nice car - always liked them from a while back. Wanted to get back into Rover ownership after a 91 214Sli and a 97 218VVC Coupe (both victims of K series head gasket failure - the coupe went with quite a bang I recall :-o )



So apart from polishing out the scratches which is kind of fun, I have this problem with the brakes. The pedal sinks when you apply the brakes, they bite then fade off as the pedal sinks. If you take your foot off the brake and then reapply it they'll be firm again for a bit then the pedal starts sinking again.



I assumed master cylinder failure since with the ignition off, pumping the pedal results in a rock hard pedal with no travel I guessed vacuum ok therefore good servo, vacuum hoses, one way valve etc. Having done about 400 miles in it with the brakes as they are there were no leaks, evidenced by no drop of fluid in the reservoir and no tell-tale puddles under the car and/or around the calipers. There's still no leaks.



So I got a replacement master cylinder (just a scrapyard job from eBay) and just finished fitting it. Bit of a bugger to bleed but it got it more or less done so I had brakes again, took it for a spin - all seemed well. Then I went round and bled again for luck, took it for another spin and the pedal is sinking again.



Now I bled the cylinder, I've put about 2 1/2 litres of brake fluid through the system, I bled each circuit independently, rear first then opposite front and just can't understand what I might have got wrong except that my initial diagnosis was not right.



Anyone got any ideas what this might be? It's an ABS model by the way....



So this is my first post, hope I've got the etiquette, place right - tell me if not!



Cheers

Matthew.

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