04 sportie - Replaced rear pads after brakes started to feel soft and poor grip. Blead the line after brake became spongy again and still have same problem. After some use in the twistees they get spongy. Take reservoir cap off and the rubber boot in the cap is deformed. After straightening out the boot and pump the brakes they are okay again for a time and then get spongy again. Removing cap and the boot again is deformed. What might be the cause and fix for this? Is the rear master cyclinder bad?