I just put on a new front tire. After reinstalling, the front hydraulic brakes took many squeezes to pump the brakes back up but eventually did. A few minutes later I was moving the bike out of the garage and the brakes worked. Then they suddenly didn't and the brake lever pulled to the handlebar with no resistance and has stayed that way. So I start to bleed the system. I have bled brakes my entire adult life and have never had any problems. Temperature is around 40 degrees. I cannot get the brakes bled after several attempts. Just won't pump up. When I squeeze the brake lever fluid does come out the bleed nipple. I see no bubbles. AFAIK I didn't do anything to the brakes except force the pads apart a little. There was adequate fluid in the reservoir to begin and I've kept it topped up as I tried to bleed. Would cold weather affect brake bleeding somehow? Any suggestions? Has my master cylinder gone bad? Thanks for reading.
Edited to say this is an 04 Shadow Aero VT750
Take the wheel back off and check the caliper pistons. I'd bet they are stuck up in their bore. Tip ...always clean the pistons before forcing them back in the bore. If they are stuck you might as well do a caliper rebuild.
If the brake caliper pistons were stuck it would still have a solid pressure on the lever, just the calipers just won't squeeze the pads.
That is pretty strange that the pressure went away just by retracting the pistons. I have done that hundreds of times myself on cars with no loss of pressure.
My older bike manual (1983 750) shows putting a spacer on the handle bar when you bleed at the lever to keep the lever from going all the way back. Maybe to keep the master cylinder piston cups from getting into old dirt and corrosion inside the master. But I don't see the same warning on the newer manuals I looked at.
I mentioned that in case the old master cylinder piston cups have been damaged from pushing them all the way to the bottom of the bore. -??? Just a thought.
Maybe a vacuum bleeder would be the best way for you now.
When you pushed the pistons back in did you have the cover off the master to see the fluid level rise? And did you clean out the chamber with a suction or rag? Maybe you got an air bubble in at that time. One bubble will cause you grief. I will many times gravity bleed the system to get a good steady flow. Just open the bleeder on the caliper a turn and keep filling the master so it never goes dry and let it run for several minutes. Then do the pressure bleed operation as normal. Hope that works.
I'll bet you pushed debris back into the master and you've got a plugged return port to the reservoir. Is your fluid nice and clear or is it yellow or brown in color? Time to at least clean and check it.
Swifty, I did not take off the master cover until I started trying to bleed the system. The fluid was dirty and I did gravity bleed until I was getting clean fluid. It took quite a while and I assumed it was because it was cold and the fluid was more viscous. Tried bleeding after that with no luck.
Simile, pushing debris back into the MC is what I'm currently thinking. Tomorrow I'll take the thing apart and attempt to clean. Not sure about the return port being plugged but I'll check that when it's apart.
I will say that when bleeding I wasn't getting a very healthy squirt out the caliper when squeezing the brake lever. I've never bled the brakes on this bike so I don't know how much I should have been getting. Just remember thinking that is wasn't much. If it was a abnormally low amount I suppose that would indicate a bad master cylinder. IDK. Just hope I can get it apart without destroying something. Just one thing after another
A trick I have done on cars and my bike also is to loosen the bleeder a turn and put your finger over the end of the bleeder to act as a check valve. You could wear rubber gloves too. Have someone else pump the lever slowly and keep the master full of fluid. As the fluid is pushed out past your finger you can feel air bubbles and the weaker stream will get more forceful, and you know you have a good solid stream of fluid. Then give it a couple of pressure /release bleeds, and it should be good to go if the master can hold pressure.
I've ordered a rebuild kit and in the meantime disassembled the master, cleaned it the best I could (it was really dirty) and reassembled. Now I can pump it up but it won't hold and the lever pretty quickly falls to the grip. Pretty sure it's the MC and hope the kit takes care of the problem. Appreciate all the help.
Don't overlook the tiny port in the bottom. It is a compensation port that if it is plugged will hold brake pressure on the caliper and not release it back into the master.
This is my clutch master, but the brake is the same and may have a metal shield sitting over it.