Brake pedal will not firm up

[weazgti]

The brakes on my mk2 are rubbish with odd pedal feel. I changed the rear caliper for mk3 ones and have bled it through on the floor with compensator valve clamped for good measure about 5 times now but it still feels odd when I try moving it. With engine off the pedal is solid however as soon as its running it feels spongey if I press it slowly but firm briefly if pushed quickly. Feels ok then sort of loses feel.
There is no air in the tube when I'm bleeding (I used a one man bleed bottle but did it the old fashioned way to be sure) and I started furthest away first and worked round.
In thinking maybe the master cylinder but have never had one go before so don't really know if its that or not.
What does anyone else think?

[weazgti]

Bled again for bout an hour with engine running and handbrake off and the fluid is perfectly clear.
Anybody?

[Gti_Jamo]

It does sound like a master cylinder fault. If your adamant the system is free of air. All your brake pipe joins tight? Even if its not leaking fluid a slight leak in the system can draw in air and disrupt the bleeding process.

[robiwill]

just had the same thing when we replaced loads of brake pipes on a valver so changed the master cylinder and cured the problem. seems that by bleeding it you push the seals through on the cylinder and it was the original I would suggest trying the same

[weazgti]

If I got a new master cylinder would the same thing happen if pedal goes to floor? Or is it just old ones that need to bled more carefully?

[Gti_Jamo]

Never let them go right to the floor. It should bleed fine pressing it 3/4's of the way down.

[weazgti]

Ended up taking off the caliper I changed, jammed someting in so the piston wouldn't move and bled it lots with the bleed nipple pointing straight up. More air came out and I have some pedal feel and brakes but I also noticed that the seals where the reservoir sits on master cylinder are wet if I move the resevoir around.