Bad master cylinder?

[michael everson]

I have been trying for 4 hours to bleed the brakes on this new daytona build. The fronts bled right away. The rears with the 5/8 master will not bleed. Pedal goes right to the floor. I took the master apart and didn’t see anything wrong. I have no leaks. I keep getting bubbles from the calipers. Not just a little. I probably put 2 big bottles of fluid through the system with no luck. I see a lot of bubbles in the reservoir when I pump the pedal. Where is that air coming from? 70 plus cars and have never run into this before.
Any ideas?
Thanks mike

[wallace18]

I have been trying for 4 hours to bleed the brakes on this new daytona build. The fronts bled right away. The rears with the 5/8 master will not bleed. Pedal goes right to the floor. I took the master apart and didn’t see anything wrong. I have no leaks. I keep getting bubbles from the calipers. Not just a little. I probably put 2 big bottles of fluid through the system with no luck. I see a lot of bubbles in the reservoir when I pump the pedal. Where is that air coming from? 70 plus cars and have never run into this before.
Any ideas?
Thanks mike

Sure sounds like a bad master Mike. I have had trouble with the rears on my last 2 builds. Ended up using Marsha to help bleed them the old fashion way

[michael everson]

Yeah. I tried vac bleeding and the old way. I did get a hard pedal for a couple of strokes then it goes away. Its only the back.
Mike

[mikeinatlanta]

Not familiar with your exact master internal layout, however, air ingress at the master is common with a plugged compensating or intake port. Depending on design it could be something else, but basically something allows it to generate a vacuum on the return stroke that overpowers a seal only designed to hold pressure from the other direction. If it can't pull freely from the reservoir it pulls air from around the seal. Can also happen with too small or a kinked line to the reservoir.

[michael everson]

Mike. Sounds plausible. I am using the small master supplied by FFR and using the hoses and res. that they supplied. I tried a new master with the same results. I also tried vacuum bleeding which should eliminate the problem you described? I am at a loss. 3 more hours this morning with nothing but bubbles.
Mike

[donshapansky]

Mike have you checked the bleeder position to the piston reservoir, if the bleeder is pointing down it will not bleed the air out. I found in the fine print the caliper has to be positioned in the 3 or 9 0'clock position. That means you have to unbolt the assembly to rotate into position. As soon as I did that bubbles came out and I had a hard pedal. I'm assuming you have the single piston caliper on the rear brakes.

[mikeinatlanta]

Mike. Sounds plausible. I am using the small master supplied by FFR and using the hoses and res. that they supplied. I tried a new master with the same results. I also tried vacuum bleeding which should eliminate the problem you described? I am at a loss. 3 more hours this morning with nothing but bubbles.
Mike
Mike,
Have you gone to the point of isolating the master from the calipers and are you certain the air is at the master? Isolating is a pain but it takes the calipers out of the equation.

I have an air ingress issue on a motorcycle clutch that I isolated using a rig made with a short length of rubber hose rigged to fit up with a banjo in lieu of a slave. Once the hose was bled I still get air ingress with multiple master cycles. With no slave seal in the picture the master is all that's left. Still haven't fully fixed it yet either (brembo master).

[idf]

I had a similar issue on completely different car (Lotus 7). Unable to bleed but with no fluid leaks. I was able to isolate the problem to a rear wheel cylinder, but it persisted after replacing the cylinder. I then replaced the hard line from the T at the rear of the car to the cylinder and all was fine.