Brake master cylinder workings

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4cambob
Posts: 106
Joined: Sat Sep 24, 2016 9:19 am
Dino: Fiat Dino 2.0 Coupe

Brake master cylinder workings

Post by 4cambob »

On my 2l coupe I removed the two pistons and replaced the seals on my master cylinder as the bore seemed perfect but the rear brakes were not working, but I cant figure out if something is amiss with the master cylinder. When I installed the second piston it had to be done against the resistance of the two springs from the first piston.And even when installing the last two springs the second piston barely seems to go further down the bore than from its fully "open" position.
My understanding of the design is that the first piston (front brakes) can go all the way to the "rest" position without resistance as it has its own two springs as well as the two springs from the second piston forcing it there. However the second piston has to over come the resistance of the springs of the first piston with the same size/type springs of the second piston so it ends up somewhere very close to if not at the end of its "open position". I would further guess this design is to allow more bias to the front brakes.
when I try and bleed the system the fronts are successful and the front brakes work, however the rears will not bleed as the second piston in the master is never in the rest position so cant displace the fluid when the foot pedal is depressed. If I open up the bleed nipple on the rear calipers the fluid will slowly drain out as there is no blockage but does not come out with the help of depressing the pedal.
I would guess that the second piston should sit approximately an inch or so further in from its "open" position as that is the approximate length of the channel in that piston but I cannot work out why my one only seems to sit in the "open" position.
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