Master Cylinder operation question

Hello all,

I just recently rebuilt my master cylinder on a 1980 Santana 109. It has the dual circuit with servo system. My question is this. Does the master cylinder port fluid to the front and rear at the same time or is it progressive. I notice it being progressive and seems to port the same amount of fluid but I am fighting the shuttle valve not letting fluid get to the back after the first initial pump. I do have a new shuttle valve coming. If the master cylinder is porting progressively could the front be keeping the back side from porting fluid through the shuttle valve? Thanks for your time.

are you fighting shuttle valve or not got front part of master cylinder bled at m/c ,if shuttle is closing you could try opening a front bleed nipple at the same time ,normally shuttle valve doesnt give trouble if your careful

jamesmartin said:

are you fighting shuttle valve or not got front part of master cylinder bled at m/c ,if shuttle is closing you could try opening a front bleed nipple at the same time ,normally shuttle valve doesnt give trouble if your careful

Click to expand...

The shuttle valve stays open for the front brakes but closes off the back break ports. I am still trying to figure out if this is because the rear break port of the master cylinder pushes fluid after the front or just a bad shuttle valve. I have tried bleeding the brakes and can get the front to pump up but not the back after a couple of pressings of the brake pedal. I believe the culprit is the shuttle valve, that is why my question is does the master cylinder port fluid out of both front and rear ports at the same time or is it progressive. Thanks for the help.

both at the same time .are you sure valve is shutting off rear ,have you actually got good fluid from m/c for rear ,does fluid in m/c res splash back when you pump pedal

jamesmartin said:

both at the same time .are you sure valve is shutting off rear ,have you actually got good fluid from m/c for rear ,does fluid in m/c res splash back when you pump pedal

Click to expand...

I get good fluid out of the master cylinder. I have bench bled it and bled it on the vehicle. With just the lines hooked up the master cylinder pushes fluid out of the front port first, then the back side. When I put the shuttle valve in the equation it starts off the same but the valve will block off the rear. My theory is this is because the master cylinder ports to the front half first and the rear side can't return the valve to center. Thank you again for the help.

try with a front nipple open at the same time

jamesmartin said:

try with a front nipple open at the same time

Click to expand...

I will give that a try in the morning. Thank you

you have got it piped up correctly ?

jamesmartin said:

you have got it piped up correctly ?

Click to expand...

As far as I can tell. Lines only fit in one position. Port furthest from the servo on the master cylinder feeds the back and port closest feeds the front. I bought it in this configuration. Just been chasing brakes since I got it. Front brake hoses were rotted from the inside and plugged. Changed all of those and put new seals in the master cylinder, pulled the shuttle valve out and cleaned it then new seals there. Wheel cylinders are in good shape.

m/c is correct ,though i cant remember the pipe order for valve as said opening front should equalise pressure

jamesmartin said:

m/c is correct ,though i cant remember the pipe order for valve as said opening front should equalise pressure

Click to expand...

I will give that a try in the morning. If it equalizes and works properly then that would suggest a faulty master cylinder correct? Thanks

i was thinking more the valve since you have good fluid from m.c, there supposed to work like that(with pressure only on one side ) but you normally dont get an issue bleeding

Thought I would give an update. I swapped out the shuttle valve and that seems to be the fix. I am just fighting air at the moment but I have more braking power than before. Thanks for all the help.

Does the master cylinder not have different bore sizes for front and back ie more fluid pumped on each press to fronts to give bias to front brakes and their larger cylinders?