The hole X is to allow the fluid or air to pass up into the reservoir when the stroke returns.
The main charge is pushed by the piston H, seals K and F, past bleeder valve ABC. Then returning, it is pushed by the wheel cylinders past ABC and up into X.
It works if all the parts are there.
I understand how it works as a piston under normal circumstances. Please explain to me how it works as a pump, when you are bleeding the brakes and the fluid does not return to it but must all come from the reservoir.
I donāt have one apart so I can only guess at the diameters, so donāt take any of these numbers as inerrant in the autograph, but anyway here goes:
The rubber washer A is letās say 7/8" diameter, so letās say .601 square inches in pressure area. So are the seals F and K.
Seal A is pushed up against the end of the bore by the spring D and metal body B and effectively blocks flow, but rubber cup seal C is smaller cross sectional area, and is not held closed by the spring D. Letās say about 1/2" diameter or .196 square inches. Think of it as a flexible cup or floppy hat.
Step on the brake pedal with say 10 pounds of linear force and with a 10/1 moment arm you may get 100 pounds of linear force at the master cylinder shaft. Fluid pressure at the piston J is thus 60 psi.
60 psi at the seal C opens it around the outside, the brim of the hat, and you get fluid down the pipe to the cylinders.
Wheel cylinder pistons are 1.122" (I have that in my notes from when I made new ones out of stainless) so .988 square inches. 60 psi there will cause them to exert a force of 59 linear pounds of force on the shoes.
Release the pedal and the wheel cylinder pistons force the fluid back at 60 psi.
But the seal C is a one-way seal. It does not open at this point, the floppy hat is pushed up against the metal body B, no backflow there.
Instead, seal A opens and lets fluid back into the chamber around spring D.
Any air should go up vent X when the seal F is fully retracted.
So fluid goes into the pipes through the floppy hat C, and air is sucked back out through the end seal A.
The bleeders are hexes. I open the bleeders about half a flat or less, just enough to let fluid and air flow out through the clear hose to the jar; fluid also seals around the threads and stops air entering there. Itās atmospheric pressure on the outside, where it is 60 psi inside. The hose is 1/4" (or was it 3/16"), larger cross sectional area than the gap in the threads, so all reverse flow takes place there out of the jar. No backflow of air through the threads.
The manual says open the bleeders 3/4 turn but thatās too much. I donāt have an explanation of that, unless perhaps the original bleeders had a rubber coating on the threads? Some of them had a ball seal in there, rather than a pointy end, maybe thatās related?
I hope I have explained the system reasonably close to right, but again Iām not sure of the diameters inside the single master, and didnāt measure my pedal arm, so donāt quote my pressure numbers.
You can convert to Pascals if you want.
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