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How do Master cylinders work?

KEITH KRAUSE

KEITH KRAUSE

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I must admit I don't know much more than:you squeeze the lever,which pushes a piston in the master cylinder,which creates pressure to the calipers. If you look in the bottom of reservoir,you can see a very small hole,what is it's purpose? When you release the lever, how does the fluid re-enter the reservoir? When you pull in the lever and there is good pressure/feel,but when you release the lever it returns out very slowly,what is causing this? Also I've read here that if you install a master cylinder with a smaller piston you can increase lever pressure. How can a smaller piston that should move less fluid,increase the pressure? That would be like saying putting a single disc master cylinder on a dual disc set-up will improve the pressure/lever feel?
A lot of questions,I know,but thanks for any help! KK.
 
I've got my front master cylinder off to rebuild it now. I'll let you know in a few days about the physiology of the thing.

The business about the piston size has to do with the PSI one can develop with the same amount of grip strength: a smaller piston will allow the same amount of grip to develop greater PSI from the master cylinder. The problem here is that the displacement of that smaller piston might not be enough to allow the brake calipers to engage the rotors.
 
I did just actually put a master cylinder from a dual front disc on a single disc. I am noticing no real difference in pressure. I would guess that if you want more pressure you'd have to verify piston sizes. I am thinking the one I had and the one I got are the same size.
 
When you squeeze the lever you activate a plunger inside the "cylinder" of the master cylinder. This plunger has one main real seal (so the fuild doesnt leak out all over the place) and the inner seal is the one the one that creates pressure to activate the system. The lever returns to its original position because a spring in the cylinder that is pushing the plunger back out after braking, this works with the fuild and lack of air in the system (important to bleed system properly) to pull the brake piston back into the caliper and alieviate the pressure pushing the pads on the discs. The second (inner) seal has a one way check valve built into it. this allows fluild behind the valve to pass into the first chamber while the brake lever is returning to its original position. The little hole you see allows the fuild in the resevoir to enter the rear chamber and pass through the one way valve into the braking system. This one way valve only operates as long as there is a need for fluid in the front chamber (to activate the brakes). once a system is properly charged (no air and full of fluid) the check valve serves as constant pressure equalizer without letting air into the system, it aloow more fluid to enter the system. thats why when you pump a brake lever it gets a bit harder to pull in each time. each pull presurizes the system and then the return of the plunger allows a bit more fluid into the chamber leaveing you with a fairly frim pull on the lever. air compresses a lot easier than any fluid and a lever pull on a braking system with air in it will feel spongey because you are working to compress the air in the lines instead of transmitting all the hydraulic force to the caliper pistons. you can increase piston pressure simply by looking at the surface area of the plunger face. Its the same hydraulic principle with floor jacks and hydraulic rams. Work (in this case hydraulic) is a funtion of force times distance. with a smaller surface area on the plunger it will require less force to squeeze but more lever travel to create the same force. The theory of 2-4-6 piston calipers take advantage of the surface area component. they use smaller pistons but use more of them to use the same braking force but apply it over a greater force over the surface of the discs.

hope this helps
 
Mark said:
The theory of 2-4-6 piston calipers take advantage of the surface area component.
Click to expand...

Most of the GS models use single piston calipers up front. The caliper slides on pins to draw the passive side against the rotor as the pads wear.
 
Hi Mark,and thanks for all replies! My only problem with the front brake is the slow to return lever. The feel at the lever is good and hard braking only requires about 1 1/2'' of lever. I have braided lines and run silicone DOT 5,
I have used it for 23 years. I inspected the calipers for wear,pistons are within spec's,I replaced the axle bolts the calipers slide on,re-greased,new seals and there are no leaks. I don't have a caliper tool that will reach far enough into master to measure the bore,but it looked OK,no wear marks or roughness. I can't buy a new one,discontinued. Replacing the piston/cup set 6 years ago fixed it for 3years,now this one is possibly failing. Also,when I bled the system I could pump the lever several times and see no more bubbles,but just when I thought it was done,a small trail of tiny bubbles would show up. This went on and on,you could almost count
the number of pumps and guess when the tiny bubbles would be back. I also noticed tiny bubbles coming from the very small hole at the bottom of reservoir(is that right?)
With no leaks,how can air be entering? I'm going to try again to bleed ,but I bet the same thing is going to happen. Even with those tiny bubbles I could'nt remove,the lever and brakes feel good,it's that slow return lever that bugs me. The lever returned just fine until 6 months ago and seems to return better one week then get worse the next. What do you think? Any tips on how to get ALL the air out? Or do you think I've gone through another piston/cup set? Thanks for any help! KK.
 
well one per side works out to two in total :lol: . very ture bill, and note how large the piston outer diameter is compared to the smaller units offered on todays "high tech" vehicles (at least more high tech than my 750). Cut the last post short cause i had to go to class, could go on for pages on hydraulics.

Mark
 
Hey Keith,
if you take the whole assembly of the handle bar (just undo the banjo nut and drian the fluid into a container) and take the lever off. you will be able too see an internal snap ring holding the whole pulnger assmebly in. take this ring out, just start one end and it will be half out. just use small skinny tools (the size of a tool is directly inverse to its replacement cost). you may be getting a small air leak into the top side (where the lever activates the plunger). there is a dust boot that you will have to remove to get the whole deal out. This boot may be the cause of your problem as water may rust inside the boot causeing a bit of corrosion or back pressure of air being pushed into the system. the whole assembly is easy to rebuild and you can visually inspect all the seals and the cylinder walls while it is apart. i did mine and it only took me a few hours to have it all apart and inspected (be sure to clear the passage ways of the one way valve system). plus its an excuse to drain and replace the fluid for a fresh riding season. With the snow we had here in southwestern ontario i wont be riding soon, and you could easily have it done before your next ride.

Mark
 
Hi Mark. You said to be sure to clear the passages of the one-way valve. Is that the very small hole I see at bottom of reservoir? If I saw bubbles come from it, it must be clear right? Unless I find something,I guess I'll buy another re-build kit,after I try bleeding. Thanks for your time! KK.
 
Keith if you look into the bottom of the fluid reservoir you will see two holes, one is the supply hole(the bigger one)and the other one that is generally closer to the banjo bolt is the return hole.
I think you may have some obstruction of the return hole which is why you may be getting some air trapped in the system and would explain your lever being slow to return sometimes.
Dink
 
Take dinks advice and check those two holes for obstructions. The small holes I was talking about are located on the actual pulnger itself. They are visible on the one side of the one-way value (not sure if there are 4 or 6 holes...). The other end of these holes is covered by the 4 or 6 sided metal spring flap that acts as the valve part. I used a fine pipe cleaner and cleaned out these holes, be sure not the bend the flap part too far of the plunger face. If the tabs are bent too far they will not return to the plunger face and wont seal properly. They are only meant to open with required (demand for more fluid in the system).

Mark
 
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