Please help. Made a video of my brake-dragging problem.

[Normk]

The term "pump-up" or without the "-" as I can't recall the correct form, refers to a separate action involving the main sealing cup. When the brake (or to a lesser extent, the clutch) is applied, it is accomplished, as we all know, by forward movement of the master cylinder piston which displaces fluid until all clearance is removed from the system.

From that point, further application of force to the piston builds pressure with the system. If there is excessive clearance within the system, the piston will reach the limits of travel and no further pressure will build.

If we relax the lever or pedal, the master cylinder piston returns to battery (the resting position) and reapplication results in the same limited or no action regarding the slave cylinders (clutch servo, brake wheel cylinder, or brake caliper).

In order to allow for emergency application by moving additional fluid forward in order to take up clearance, designers create a lip type piston seal which has some side clearance between the body (main diameter) of the sealing cup and the master cylinder bore. In this fashion the rubber seal is in the form of a tapered cup with the lips of the open end spead against the surface of the bore.

In normal release, the cup and piston simply return to battery as already stated but in a rapid release the inertia and density of the fluid cause it to resist rearward movement being applied by the sealing cup. In this condition, the sealing cup's lips flex inward allowing some fluid to by-pass the cup in the forward direction, so that the fluid moves into the area ahead of the piston before the piston clears the compensating port.

Rapid partial release and application of the linkage cycles the piston forward to create pressure/take-up clearance and then rearward to by-pass more fluid. With each repetition the piston's forward movement acts on an increasing volume of fluid forward of the piston sealing cup so that the piston is able to apply more pressure due to the reduction in clearance.

I think everyone will recall the effect of short rapid pumping which moves the lever outward or the pedal upward at the point where application force is stopped by the resistance of the brake?

If one experiences a malfunction such as forgetting to bleed the air from the system this pump-up action can be a life saver because it allows one to literally pump up the system pressure sufficiently to obtain some brake application. Slow or normal application rates will not accomplish this effect.

Similarly, if one does repeated hard braking with contaminated brake fluid which boils, this may save the day.....or not so change the stuff every year. Please!

HIH

Norm

[Steve]

I finally figured out what you are saying here...HIH = Hope it Helps.

Wow, I'm getting old.

Cowboy...I've been wondering the same thing. Guess who doesn't carry a cell phone or text message? Hmmm, maybe I AM getting old.

I'm not getting old, I AM old.

I'm tired of showing off my ignorance of "texting" lingo. I figure if it's not important enough to type it out, it's not important enough to worry about.

Thanks for the translation, cowboy.

Norm, thanks for the write-up. Now we just need to get some pictures attached to that tome and get it posted in the "Brakes" section of BassCliff's library.

.

[Normk]

Here's some more brake master cylinder stuff which might be useful to someone:

Everone knows how disk brakes self-adjust because the pistons do not return as far on release as the fruction material wears down. Of course this is why the fluid level drops as the pads wear and why we need to suction out some fluid from the master cylinder reservoir before pushing the caliper piston(s) back in order to install new pads.

Likely should make sure that everone understands the action of the caliper's square sealing ring and the need to make sure that the piston is free.....

I'm sure that most people have also noticed the difference in shape between the master cylinder sealing cup(s), caliper seal rings, wheel & slave cylinder seals and likely know why they are differently shaped?

Here's a point which can save some grief:

Sometimes, after replacing brake pads or otherwise having pushed a caliper piston back or when bleeding brakes, one may have trouble obtaining a hard brake. In other words, the lever or pedal has a spongy feel and full pressure can only be obtained by pump-up. Air is often blamed but the cause may lie elsewhere.

In normal braking action, the master cylinder sealing cup moves through a limited range and never beyond. If there is water present in the system it will drop out of solution under colder temperatures. Remember your college physics? The solubility of a gas in a liquid, solid in a liquid?

The water will collect and remain in low areas within the braking system, one of which is along the bottom of the master cylinder bore. Hope no one will advance that silly myth about brake fluid not needing to be changed or the one that silicone brake fluid is better!

If the water lies within the stroke of the piston cup, it will be pushed forward where it will remain, corroding & pitting the master cylinder bore. When something happens to require an increased piston stroke, the sealing cup will slide forward being expanded by increasing pressure, over the corroded/pitted area. The contact will wear/abraid the sealing lip surface of the cup compromising its ability to seal to the bore. It can require only one stroke into a corroded/pitted area to fatally damage the sealing cup, especially when combined with normal wear to the sealing lip.

When one next tries to apply light braking, the cup is unable to seal effectively so that fluid leaks back, past the sealing cup and the lever/pedal sinks, perhaps all the way to the end of the bore and no braking pressure is created. A rapid application, sometimes requiring some pump-up to move the piston cup back to a smoother area, with hard application will often create enough pressure on the cup to affect a seal.

Most of us have experienced a brake lever of pedal which gradually sinks unless it is applied quickly and hard. Some have experienced a clutch which will disengage if the lever is pulled quickly but the clutch will begin to gradually apply as though one was slowly releasing the lever. This can be a quite humerous condition if one observes a riding buddy dealing with a misbehaving clutch of this type.

Old timers have learned not to stroke a used master cylinder further than it is used to do in order to avoid this happening. The obvious problem though, is that the system may let one down unless the master cylinder is repaired or replaced.

It has always troubled me that so many people will avoid doing a valve adjustment or cleaning carburetors because these are complicated and intricate operations but the same people will fly at a brake job with no understanding of the systems. What's the worst from a too tight valve or flooding carb.? Death is unlikely

HIH someone. I will see what else pops into mind.

Norm

[Smellii]

Thanks, Norm! I know the importance of understanding how my braking system works; now I actually understand how my braking system works.