Several reasons. Your rotors could be warped. Its possible the proportioning valve isn't working correctly. Try a different type brake pad material. Make sure your wheel bearings are not too loose.
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have you cleaned and lubed the caliper rails?
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One of the most overlooked tasks when performing brake maintenance is the floating pins that the caliper mounts to. Make sure they are clean and well lubricated.
It's rare....because usually the master cylinder will allow pressure to bleed off. Not the other way around. But if you have replaced the calipers, this is the next thing I would check. I suppose it could be sticking.
have you tried the proportioning valve. sometimes dirt will get in there and clog the returns.
It is time to replace the brake calipers when they begin sticking closed or do not close all the way. You might also have a caliper that locks completely which needs to be replaced.
Check if the calipers are sticking.
seized calipers?
I had one do this and I had to change the brake calipers. Problem was gone.
the pistons inside your calipers are sticking
The brake proportioning valve may be sticking, but this is very uncommon. Did you lubricate the slider pins on the caliber or replace them if they are rusted or corroded? Check all contact points and clean them off throughly. Apply brake lube to all contact points.
you need to change the spring in the clutch cylinder ,you find more info on at alfa .net
MORE THAN LIKELY THE RUBBER HOSE HAS SOMETHING IN IT. REPLACE THE HOSE SHOULD BE AS GOOD AS NEW.
Check for sticking calipers.
Defective Master cylinder?