No Front Brake, No Pressure to Lever
- Thread starter J-Rod10
- Start date
Tune-A-Fish
BOTM LOSER Proudly Deplorable
J-Rod10 said:
Has to be bypassing at the master try putting an 8mm ? cap bolt and nut (not banjo) on the line and see if you can get pressure at the master, if yes you might try opening the bleeder and letting fluid drain into a clean plastic cup while keeping the res full just to eliminate a bubble.
Before you get carried away, try bleeding the system by just cracking the banjo bolt at the master, squeezing the lever, tighten the banjo, release the lever, repeat. This makes a giant mess, so be prepared with lots of paper towels, but 99.99% of the time this works almost instantly. If the master is good, this should work, and bleeding the rest of the air out should be a piece of cake down at the caliper. If you get no satisfaction, remove the line and banjo bolt from the master and use your finger as a one way valve over the bolt hole. This will work in just a few pumps (likely on the first pump you will detect that you are making progress) absolutely 100% of the time. If that does not work, there is an issue with the master.
If you seal up the output of the master with a bolt, I expect it will be very difficult to remove the air in front of the piston and have a certainty of knowledge as to what is wrong if anything, though even with the air trapped inside, you should get some indication of pressure due to the now much smaller volume in the space in front of the piston. Keep in mind that the one way valving action of the master is extremely effective, and that it works just as well with air as it does with fluid, so any air in front of the piston will never come back to the reservoir unless the master is bad. The air bubbles you may see bubbling up in the reservoir are from the space between the two seals on the piston, the back seal that keeps the master from leaking, and the front seal which works as the one way valve. That portion of the system between the seals is self bleeding. Everything else has to bleed elsewhere. The problem often is that there is such a large volume of air directly in front of the piston, that it just moves back and forth with the piston, and there is not enough negative pressure to keep drawing fluid past the one way valving action because the system is mostly filled with air. Any tiny bit of fluid that is drawn in trickles down to the caliper, where it gets removed by attempted bleeding and the interior air volume stays the same. You keep removing fluid but never any air. Even if air is removed on one stroke of the master, it can be possible that when the lever is released increasing the system volume, the tiny negative pressure left after the air expands is simply not enough to draw in much fluid. This makes bleeding impossible, or at the least seem that way. Nothing in front of the front seal can back-track unless the master is bad, so if air is there, gravity will keep it there because that banjo fitting is usually the highest point in the system. The reservoir may be higher but that is irrelevant as there is not enough head pressure to push fluid past the seal to the front of the piston. Bleed the air out at the banjo, and fluid can trickle down the line to mostly fill it because the air can escape (also keep in mind that most lines are small enough that fluid resists allowing air to percolate upward through it against gravity due to surface tension. Fluid does indeed "trickle down", but with considerable resistance.). More importantly, the large volume in front of the piston can also fill for the same reason. This makes the system volume mostly filled with fluid, but more importantly, not with much air. The air is very compressible, and if too much is in the system, it is enough to be able to expand quite a lot before fluid is forced passed the one way valving action at the master and nothing happens. Replace that volume with in-compressible (also un-expandable (sort of)) fluid and the pumping action can take place and you can bleed the remaining air at the caliper. Once there is enough fluid and little enough air in the system, the master can push fluid very aggressively through it. Enough so that air bubbles in the small diameter lines are driven along with fluid down the lines (before they can bubble up back to the banjo) into the calipers where they can be purged with the bleeders.
If you seal up the output of the master with a bolt, I expect it will be very difficult to remove the air in front of the piston and have a certainty of knowledge as to what is wrong if anything, though even with the air trapped inside, you should get some indication of pressure due to the now much smaller volume in the space in front of the piston. Keep in mind that the one way valving action of the master is extremely effective, and that it works just as well with air as it does with fluid, so any air in front of the piston will never come back to the reservoir unless the master is bad. The air bubbles you may see bubbling up in the reservoir are from the space between the two seals on the piston, the back seal that keeps the master from leaking, and the front seal which works as the one way valve. That portion of the system between the seals is self bleeding. Everything else has to bleed elsewhere. The problem often is that there is such a large volume of air directly in front of the piston, that it just moves back and forth with the piston, and there is not enough negative pressure to keep drawing fluid past the one way valving action because the system is mostly filled with air. Any tiny bit of fluid that is drawn in trickles down to the caliper, where it gets removed by attempted bleeding and the interior air volume stays the same. You keep removing fluid but never any air. Even if air is removed on one stroke of the master, it can be possible that when the lever is released increasing the system volume, the tiny negative pressure left after the air expands is simply not enough to draw in much fluid. This makes bleeding impossible, or at the least seem that way. Nothing in front of the front seal can back-track unless the master is bad, so if air is there, gravity will keep it there because that banjo fitting is usually the highest point in the system. The reservoir may be higher but that is irrelevant as there is not enough head pressure to push fluid past the seal to the front of the piston. Bleed the air out at the banjo, and fluid can trickle down the line to mostly fill it because the air can escape (also keep in mind that most lines are small enough that fluid resists allowing air to percolate upward through it against gravity due to surface tension. Fluid does indeed "trickle down", but with considerable resistance.). More importantly, the large volume in front of the piston can also fill for the same reason. This makes the system volume mostly filled with fluid, but more importantly, not with much air. The air is very compressible, and if too much is in the system, it is enough to be able to expand quite a lot before fluid is forced passed the one way valving action at the master and nothing happens. Replace that volume with in-compressible (also un-expandable (sort of)) fluid and the pumping action can take place and you can bleed the remaining air at the caliper. Once there is enough fluid and little enough air in the system, the master can push fluid very aggressively through it. Enough so that air bubbles in the small diameter lines are driven along with fluid down the lines (before they can bubble up back to the banjo) into the calipers where they can be purged with the bleeders.
Pickinfights
New Member
Thanks JP. You have restored my faith in physics. Been fighting these brakes for days. Now I can take the shotgun out of my mouth. ;D
Powderfinger
Been Around the Block
A vacuum pump is great for stubborn to bleed motorcycle brakes and hyd. clutches. Take your bleed screw out and carefully put a little Teflon tape around the threads to keep from sucking air around the threads.
