[hogfarm]

Originally Posted by dsraven

1-there should be free play at the top of the pedal so the master cyl has room to return fully. check this by removing the master cover, have a buddy lightly step on the pedal while watching the fluid in the res. when the pedal is first pushed there should be a little fluid pushed back into the res creating a small fountain. that happens because the piston, at rest, is behind the little holes in the cyl that allow fluid to enter the bore or return to the res. there should be a little fountain of fluid, then as the pedal is pushed a little further the fountain goes away and fluid is moved down the brake lines. if no fountain appears then maybe the adjustment on the small pushrod from the booster to the master is made too long, so the little holes don't get uncovered all the way, or else the pedal pushrod is too long and is keeping the piston from returning all the way.
2-the pedal should push the master cyl all the way to the end of it's travel. check this by lossening a bleeder, front and rear, so the pedal will move easily and fully, then remove the pedal pushrod and use another method to push the master all the way to the end of it's travel. compare that to how much the pedal would move the master if it was pushed all the way to the floor
3-check to ensure you have a master that is made for disc front and rear. some with drum rear would have a residual valve built into the line connection in the master.


it kinda sounds like your master cyl is not fully retracting. if you are getting no fluid at the bleeder unless you vacuum bleed. vacuum bleeding can "pull" fluid past the cups on the piston so that part would make sense sorta, if you get fluid when vac bleeding but nothing really if pushing the pedal. the piston is not coming all the way back into the master to allow fresh fluid to enter the bore so all you have to work with is what is already in front of the piston when you start.this will lead to issues as the brakes heat up from use because the fluid expands and applies the brakes for you without you pressing on the pedal. try loosening the master mounting nuts a bit so there is some play there and see if that helps.if is does then that could be the problem. does the pedal feel firm, sloppy, like it is just moving but not making pressure? does it get firm before bottoming out? some systems that get modified don't have a full travel for the master cyl because the attachment point for the pushrod is closer to the pivot point for the pedal arm. ensure you have a full stroke for the master-step 2.

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