bleeding issues

[Guest]

How long should it take to bleed all the air out when doing the front brakes? Ive never been able to get the mushy feeling out. Ive put in a master cyl from a later model suzuki that is in very good shape. New seals in the mc and calipers. new lines. Used a syringe to pull the air out and it seems endless. There are no leaks. Ive tied back the lever overnight and got some results but not firm. Im thinking about getting a newer, more model specific mc if i cant get it right. I spent an hour and a half bleed ing it w/o much change.

[azr]

Using the syringe method you should be done in minutes per side. Just so I'm clear, you're talking about your 1100 and the front brakes, right? If it's the rear you'll never really get a firm feel to it. The front should be able to get pretty solid even with rubber hoses. The biggest improvement to brakes I've done is changing over to stainless steel lines, it really made the brakes feel firm. Now I've never replaced the old lines with a new rubber ones but they should make a big difference as well.

[snicrep5]

yes its the 1100. I just changed to the ss lines to see if that was the problem but no change. ive got a used master cyl from an 87 gs550 but its appears to be in excellant shape. Im about to try another ms.

[Normk]

No time to post bleeding suggestions as the subject has been extensively covered elsewhere but regarding the practice of tying back the brake lever or loading the pedal in an effort to remove air:

The solubility of a gas in a liquid is inversely proportional to temperature which is why trout, who live in cold water can do nicely with small gills but warm water fish such as goldfish require large gills. Cold water holds more oxygen/warm water less.

The solubility of a gas in a liquid increases with pressure so increasing the brake system pressure will increase the amount of air which will dissolve on the fluid. (time, etc. allowing) Pumping up and tying or bungee cording a brake lever over night will cause some trapped air to dissolve into the brake fluid so the lever/pedal will often feel firmer after the interval. One needs appreciate two things here:

1) The amount of air which can be dissolved is quite small so a really spongy brake due to air will likely not be aleviated by this practice.

2) After the pressure is released the air can begin to come out of suspension, especially at higher temperatures such as hard braking. Hint, hint! Once you have a firm brake from this process, bleed the fluid out with new to remove the dissolved air containing fluid.

Brake fluid is cheap, caskets are expensive.

[daturat100r]

Let the new fluid sit still in the bottle for at least a couple of hours before adding to master.Shaken fluid is a pain to try and bleed.

[snicrep5]

you all might be on to something there. I am pouring the same fluid that i bled out back into the m/c. (its clean). Ill try not recycling it next.

[koolaid_kid]

For your own safety, never, never reuse brake fluid. Ever. Once exposed to the atmosphere it absorbs water like a sponge. Water in your brake lines will kill you.

[Dogma]

I've found that vacuum bleeding sometimes needs to be followed by old-fashioned pump-and-drain bleeding to finish the job. Also, it may be necessary to crack open the banjo bolt at the master cylinder to bleed out a bubble there (if the banjo end of the master is uphill from the valving).

Norm, How much air can brake fluid absorb? I know it loves water, but I've never heard of air dissolving into it. I've done the trick with tying back the lever overnight, but I never lost any lever firmness later. So I'm thinking air solubility is not significant in the type of fluid we use.

The theory behind the practice is that bubbles trapped in the lines may need a long time to travel up to the master, where they are subsequently pulled into the reservoir when the lever is released. Clearly this requires that there are no loops or other bubble traps between the calipers and master cylinder valving. Of course, with a good uphill bubble path, this practice should not even be necessary. I might even be necessary to find a temporary mount for the master cylinder.

[Normk]

I have not been able to obtain figures regarding the quantity of air which will dissolve into brake fluid but tiny bubbles do disappear as indicated by pressurizing a boiler sight glass. The debate in this one flows on but there is certainly a beneficial effect in many systems. I don't know how there would be a benefit in terms of clearing air bubbles through pressurizing the system so tend to discount the idea that holding the system under pressure has a benefit in terms of moving the air.

If air will not travel upward into the master cylinder while the system is under ambient (normal atmospheric) pressure then pressurizing will have a negative effect. If one has the opportunity to observe suspended air bubbles by use of a transparent enclosure such as a boiler sight glass, it will be seen that pressurizing to even a moderate degree shrinks suspended air bubbles to a remarkable degree. The smaller the bubbles, the less bouyant they are because the increase in bubble density is far greater under pressure than that of even silicone brake fluid. Conventional brake fluid compresses less than 1% per 1,000 PSI, if memory serves, but a large air bubble will have virtually disappeared by 100 PSI.

A technique we often use in purging air bubbles suspended in hydraulic oil is to apply a 15 to 30 inch (Hg) vacuum to the reservoir. The decreased pressure allows the air bubbles to increase markedly in size which makes them far more bouyant so that they rise to the top and escape. Any scuba divers in the group will discribe the same bubble growth phenomenon as bubbles rise in the water column.

Your point regarding banjo fittings is well taken, Dogma! Depending on orientation and just "because", air will sometimes adhere to the surface of the fitting. Tapping can help but sometimes one needs be creative.

An often overlooked factor when dealing with a spongy brake is perception. It is amazing how often one can develop tunnel vision and simply believe that a brake is spongier than it really is. I've frequently had to resort to some rational evaluation: Can I lock the front wheel at 60 mph without pulling the lever back to the bar? Well then, take a pill, Norm!

If one has a pull scale such as a heavy fishing unit, which are inexpensive, one can compare the pull required before and after a brake job to confirm that one is imagining things.

Another overlooked factor is that the brake pads may not be bedded to the rotor irregularities allowing for come "give" in the pads.

Bad wheel bearings can allow the wheel to drop sideways a small amount which can push the caliper piston(s) back further than expected. When the brake lever is applied the caliper pistons must move far enough to straighten the wheel before the pressure will firm up the lever. This requires little wheel movement before one cannot develop sufficient brake application. One can check for this effect by removing the pads and placing a steel or alumium block between caliper sides and rotor.

Tired....must sleep....

I've found that vacuum bleeding sometimes needs to be followed by old-fashioned pump-and-drain bleeding to finish the job. Also, it may be necessary to crack open the banjo bolt at the master cylinder to bleed out a bubble there (if the banjo end of the master is uphill from the valving).

Norm, How much air can brake fluid absorb? I know it loves water, but I've never heard of air dissolving into it. I've done the trick with tying back the lever overnight, but I never lost any lever firmness later. So I'm thinking air solubility is not significant in the type of fluid we use.

The theory behind the practice is that bubbles trapped in the lines may need a long time to travel up to the master, where they are subsequently pulled into the reservoir when the lever is released. Clearly this requires that there are no loops or other bubble traps between the calipers and master cylinder valving. Of course, with a good uphill bubble path, this practice should not even be necessary. I might even be necessary to find a temporary mount for the master cylinder.

[1_v8_merc]

You bench-bleed the MC yet?
(just scanned threw real quick, maybe I missed it)

[willie]

Having just read all the info presented in this thread I'd like to comment on how thankful and, in a sense, proud I am to be a member of a group (ie. the GSR) who's member's collective knowledge never ceases to amaze me. That those members take the time to share their knowledge with others is truly impressive and fortunate.
Willie

[Nessism]

For your own safety, never, never reuse brake fluid. Ever. Once exposed to the atmosphere it absorbs water like a sponge. Water in your brake lines will kill you.

When bleeding a fresh and clean system I recycle the brake fluid. Need to watch the bubbles as mentioned, but no need to throw the stuff away just because it's exposed to the air for a few min's.

[snicrep5]

I didnt bench bleed after changing the lines but i did originally. guess ill try that.

[janner]

flew through this thread, but if not mentioned , i always use a clear tube thats a tight fit over the bleed nipple and the other end is in a receiver jar immersed in the fluid , so that when you release the lever it cannot draw air or moisture back in, you still need to lock the bleed nipple on every stroke but having the tube is an insurance, also go for 30 seconds of fast pumping of the lever , then go the usual slow in/out , once finished tie the lever off and all should be good in the morning, works for me , good luck , jon

[snicrep5]

much better now. I connected a spare brake line to the m/c outlet and stuck it back in the resevoir and pumped it a while. reconnected the line and pumped it a bunch of time and cracked it open.It took a while but I got a little more air out. did it until I saw no air and did it longer to be sure. Tied back the lever and left it for 6 hrs or so. Now the lever is alot higher. Better than its ever been. Now Ill put fresh pads and it should be even better. Thanks to all.