QuestionQUESTION: Hi, I'm nearing the completion of my midget restoration and have started to bleed the brakes only to find that the master cylinder won't take fluid from the reservoir.
It's a rebuilt unit but even when disconnected from the system the piston won't retract unless it's allowed to draw air or fluid back in.
Should I just buy a new one?.
Thanks in advance, Mark.
ANSWER: Hi Mark,
I don't understand your statement, "the piston won't retract unless it's allowed to draw air or fluid back in"
That's true on ALL master cylinders. Or do you mean allowed to draw air or fluid back into the line fitting???.
When you depress the piston, it closes off the port between the cylinder and the reservoir and when you release the piston it returns which tries to draw the fluid back into the cylinder when the piston retracts far enough to open the port between the cylinder and the reservoir. On a tandem system the same thing happens except there are two pistons that close off two ports.
You may have a problem in the link to the pedal or the pedal is not retracting all the way.
Remove the master cylinder and by mouth blow into the reservoir and air should blow out both line fittings. This will confirm that the pistons are retracted to their correct position. Then check to see that there is NO load placed on the piston when it is installed. Then fill the reservoir and bleed at the lines first and then at the farthermost wheel and work you way back to the closest wheel.
Howard
---------- FOLLOW-UP ----------
QUESTION: Thanks for the reply,
Whats happening is that fluid is being pushed into the pipe work then suction is preventing the piston from returning unless you allow air or fluid into the system at a bleed nipple (or pipe union) so instead of drawing fluid from the resevior the same fluid is pulled backwards and forwards making bleeding impossible.
All the pipes are new and the calipers are rebuilt, the pedal movement is fine and there is no pressure on the piston when the pedal is at rest.
I have done as you suggested with the master cylinder and air does come out of the single line.
If left for long enough fluid will seep through all the lines to the bleed nipples but there is still no "feel" at the pedal.
Thanks again, Mark.
AnswerMark, that is why when you push down the pedal with the bleeder open, you are suppose to close the bleeder before you let the pedal up. When you do this it tries to pull the fluid back that you pushed out into the line and toward the wheel cylinders but it tends to form a vacuum in the lines and when the piston gets all the way back and the port is then open to the reservoir then that partial vacuum will pull fresh fluid down into the lines. Then you continue the pump up again and CLOSE the bleeder BEFORE you release the pedal and it will do the same again. After you do this several times there will be no air in the lines and when the pedal is released it will try to suck fluid back which it can't because the wheel cylinders and caliper pistons will not retract that easily. This is providing you used the correct sequence of bleeding each wheel as I outlined before.
Howard
