Brake Master Cylinder

[Michael M.]

I rebuilt my brake master cylinder and proceeded to bench bleed it. I got all of the air out of it but I noticed when I pushed the plunger in with a screw driver I got brake fluid shooting up from the front reservoir but not the back one. I thought I should get the brake fluid shooting up from both reservoirs. Is it possible I wasn't pushing the plunger in quick enough? I installed the cylinder on the car and with the ports plugged the pedal is good and hard, so there's no air in the cylinder. Need I worry? Thanks.

[Terry M.]

Re: Brake Master Cylinder

I believe the way the master cylinder functions is to build pressure in the front reservoir first (that is the fountain of fluid you are seeing) if the brake stroke is long enough then the rear reservoir comes in to play. This is all dictated by the position of the pistons and the ports in the wall of the master cylinder bore. Note the spring between the two pistons. That spring, if I understand the operation correctly, modulates the pressure between the front and rear brake systems.

All that says I think you simply didn't push the piston far enough during your test.

[Michael M.]

Re: Brake Master Cylinder

Makes sense. The proof will be in the pudding tomorrow. I'll be connecting the master tomorrow and bleeding the calipers. Thanks Terry.

[Terry M.]

Re: Brake Master Cylinder

Let us know how it works. Good luck.

[Michael M.]

Re: Brake Master Cylinder

I think I have a bad master cylinder. Front brakes bled fine. I can't get any fluid out of the rear brakes. I had my trusted assistant (wife) pump the brake pedal at least 2 dozen times for the rear brakes. When I break open the bleeder screws, I can't hear any air escaping, no fluid from the bleeders and I have no loss of brake fluid from the rear reservoir on the master. The fluid level remains unchanged.

Do you all agree I need a new master cylinder? I just rebuilt this one. There must be some sort of internal issue. As always. Thanks.

[Paul O.]

Re: Brake Master Cylinder

Mick

If you have bleeder ports on the master cylinder for the rear brakes with your assistant try bleeding there. If you have fluid coming out there then I would work your way rearward at each connection point in the rear system see where the fluid stops coming out that is where you may have material or air blockage. Or just a bad master cylinder.

Paul 18046

[Michael M.]

Re: Brake Master Cylinder

Mick

If you have bleeder ports on the master cylinder for the rear brakes with your assistant try bleeding there. If you have fluid coming out there then I would work your way rearward at each connection point in the rear system see where the fluid stops coming out that is where you may have material or air blockage. Or just a bad master cylinder.

Paul 18046

Well, I think it's a bad master. No "fountain" from the rear reservoir, no fluid movement when pumping the pedal and when I remove the bleeder fitting I got fluid from the front port but not the rear port. Thanks for everyones help.