Rear brake won't realese with modern Brembo master

Hi ! Last thing in my '83 Z1000j3 rebuild is the rear brake. I bought a "new" Brembo brake master from a Aprilia RR1000RRRRR (something race). And it seem powerfull enough since it will block the rear wheel. But when releasing the brake pedal it still brakes to much for the wheel to spin. I can barely move the bike around... I rebuild the caliper with new piston, seals and pads. It's a new braided hose and the new master has been dissasembled and cleaned. The original master is 13mm right ? The Brembo is 12mm... I know i should get a 13mm but the 12mm can brake it... I've tried different masters on front brakes and i know how a small one feels compared to a too big one... This feels fine, but it won't release enough. When the pedal is pressed i can feel preassure being build in the resevoir line.. Is that normal ? Isn't it surposed to draw liquid from the resevoir instead of pushing ?
I don't have the original master in working condition so i'm kind of lost... Any help whould be great!

Simon, Denmark

When its hanging up, crack the bleeder and see if there is still pressure. If so first check the pushrod adjustment to the master, make sure it has some free play. If thats good, and once the bleeder is opened the caliper lets go of the rotor. I'd say the fluid return port is blocked,or since its new, maybe wasn't machined?

along with motorheads suggestions.
i would add to check that the pins that hold the pads in and that the pads slide on don't have any grooves worn into them.

i had some that were worn and wouldn't release my pads and smoked my caliper,turned it blue :sick: .

keep us posted,

leon

+1 on all of the above.

Also when the brake is applied, the fluid reservoir is closed off from the system while the master cylinder pushes the fluid along the brake hose to activate the slave cylinder ( brake caliper ).
The reservoir is just that, a reservoir to ensure the system has enough brake fluid at it's disposal, it ensures that there is a head of fluid to prevent air from entering the system and also acts as an expansion tank for the expansion and contraction of the fluid during temperature fluctuations.

Motor Head wrote: When its hanging up, crack the bleeder and see if there is still pressure. If so first check the pushrod adjustment to the master, make sure it has some free play. If thats good, and once the bleeder is opened the caliper lets go of the rotor. I'd say the fluid return port is blocked,or since its new, maybe wasn't machined?

Word.

There is a relief hole on the inside of the master cylinder. If the the adjuster of the brake lever is set too high the piston seal will cover the hole and it won't release. My guess this is your issue since you are using not for the intended bike and you might be over extending it.