Topic: 2 Generic Brake questions

[MarshallCS]

Hello everyone


So after reading an older thread on here, I was able to find a set of 82 CB900F calipers and brackets for 10 dollars to put on my 78 750F. I plan to sandblast them and refurbish them during the winter time so they can be ready for the spring.


I have two pretty basic questions.


The first is: Why do the caliper pistons go in with the hollow side pointing towards the brake pads? To me it would make more sense to have the full, flat surface area of the piston pushing against the pad, and all that extra surface area of the inside of the piston for the brake fluid to act against. I'm sure there is a logical reason why it's done the opposite way, I'm just wondering why.




Second question: Why is there a junction point between the two calipers and the master cylinder? Would it be easier (to bleed specifically) to have a dual banjo bolt coming from the master cylinder and direct lines going to each of the calipers? Doesn't that junction point create a place for air bubbles to hang out? Again there is probably a logical reason why it's done that way, I'm just curious.


It seems pretty straight forward to refurbish these - they seem like they are in good condition. I need to get the pistons out of them, and I plan on replacing the seals, banjo bolts and the bleeders. Is there anything that I should be aware of, or watch out for as I redo these?




Thanks for all the wisdom in advance.
Marshall

[seanbarney41]

1.  The hollow side of the piston will require a larger volume of fluid.

2.  Dual banjo bolt at the mc should work fine and some feel it is an improvement and should be easier to bleed.  Yea, that junction does tend to trap air bubbles.  GL1000 junction actually has a bleed screw there...not sure about stock cb900 set up, but traditionally Honda used that junction as the brake light switch also.

[calj737]

+1 on #1
Junction allows balanced pressure with a single banjo bolt. Easier and cheaper at the factory. Dual banjos are an upgrade. Recommended as well for all the reasons cited.

[MarshallCS]

Is a larger volume of fluid on the hollow side a good thing or a bad thing. To me it seems like it would be a good thing - more pressure acting on the piston.


I'm glad that I was on the mark about the junction being a less optimal solution. It seems like it would be cheaper as well to have just two lines (stainless steel wise) instead of three.

[trueblue]

Is a larger volume of fluid on the hollow side a good thing or a bad thing. To me it seems like it would be a good thing - more pressure acting on the piston.

It is a good thing to have less fluid behind the piston.  First of all if you had it with the hollow side in, you would have a hell of a time bleeding the air out.  Also the fluid does compress a little, not enough to notice in normal circumstances, but if you add extra fluid it will compress more and may cause a spongy lever.  There is no difference in pressure by flipping the piston as it is stilll the same diameter which is all that matters

[bjbuchanan]

The hollow part of the piston could be to save weight, maybe simpler to produce, cheaper also because less material. You are thinking of a more is better type of way, you don't get more oomph from more fluid. Once all the space is taken up by filling with fluid you get the same pressure, tons of space or not.

Pressure is dicated by the size of the MC bore in the easiest way. 12mm, 14mm they all exert different pressure and volume so you gotta match the system. It isn't too hard to match components because there is so much info and working combos out there. Using the whole setup from the bike is easiest too. You won't have a "wooden" lever or mushy lever

[becken]

http://www.dummies.com/how-to/content/understanding-pressure-using-pascals-principle.html

[MarshallCS]

Thank you all for the wonderful explanations! It'll make things quite a bit easier as I work on the dual piston caliper mod!