Losing brake pressure

I’m near the end of a long road of winter maintenance, and I have just one more thing to do. I replaced my handlebar last month (XJ650 Maxim, so the dreaded buckhorn bars), and I got a new aftermarket master cylinder from chacal that sits flat on the new bars. I had reverse-bled the system with a syringe through the bleeder bolt, and I had decent brake pressure for a while. Even went out on a few test rides.

This weekend, I rebuilt my front forks, which required me to disconnect the caliper from the forks. I suspended the caliper from the exhaust header using a zip tie while the forks were off, just so I wouldn’t put too much strain on my nice stainless brake hoses. Weird thing though, after reassembling everything, I had a completely spongy brake lever. I assumed moving the caliper around must have dislodged some air in the system, and sure enough, I got some bubbles through the bleeder when I bled the brakes this morning.

However, even though I got some firmness back in the lever, it would never stay for more than a few pumps. I can’t get any more air to come out of the bleeder, just fresh fluid every time. It almost seems like there’s a leak somewhere, except there’s no fluid coming out of anywhere. If air were getting in, then I imagine fluid would be coming out.

Anyone have an idea what could be up here? Hard to imagine it could be more trapped air, especially since I did the reverse bleed, but who knows.

 

Take the cap off of your reservoir, squeeze the handle check to see if fluid gets pushed back into the fluid pool through the small hole in the bottom, if so probably needs a rebuild kit.

 

Huh, I thought that was normal though. When I squeeze the handle, there’s always a bit of a splash in the reservoir. It worked that way on my old MC as well, so I thought it was just displacing fluid until the piston cleared the fluid port in the bottom of the reservoir. The new MC is a brand new (albeit aftermarket) unit, so I would be very surprised if it’s faulty.

 

First do a conventional bleed by pumping the lever, shouldn't take much. If you are able to hold pressure without the lever travelling to the bar, that's a good sign of no internal leakage. Then tie the lever under pressure to the bar and leave overnight. Any air should migrate into the reservoir.

 

I am able to hold pressure without the lever traveling to the bar, but it’s pretty close. It feels like nothing is happening until more than halfway through the lever’s travel distance, so I know there must be more air in there. I tried zip tying the lever to the handlebar and leaving it overnight, and I experienced maybe a slight improvement.

I have now zip tied the lever to the handle while running a vibratory sander (without sandpaper, of course) from bottom to top up the brake lines. I’ve turned the bars full lock left so that the MC is the highest thing on the bike, and I will now leave it overnight again to see if this helps. Will report back if this fixes the problem.

 

There should only be a little quick "spit" of fluid when you squeeze the brakes from that hole not a stream during the entire time.

 

On new, dry systems, I usually wind up with a combination of the vacuum bleeder, vibration, etc, just to get everything started, but my best luck is always finishing with pumping the lever with the caliper bleeding into a mason jar of brake fluid. Have enough fluid in the jar so the discharge end of the hose is submerged (other end of the hose on the bleed screw). Put a little silicon grease on the threads of the bleed screw in the caliper so it doesn't suck air in around the threads. Then just leave the bleed screw open and pump away. The biggest pain is trying not to make a mess when refilling the reservoir on the master.

Basically, force the air bubbles down instead of trying to coax them up. In this scenario, have the master tilted up so the hose end is higher.

Also make sure the adjuster screw on the brake lever is correct, so you allow the master cylinder to fully retract each time. But yah, simple systems, but they can kick your butt sometimes.

 

Alright, so it is definitely spitting only a little bit when I squeeze the brakes. I think it’s holding pressure just fine.

It’s funny you mentioned the adjuster screw, since I may have had the opposite problem. Felt like there was a little much play in the lever, so I just tightened it down until it’s just touching the MC piston when at rest. I know when riding you’d prefer to have a small amount of free play in the lever, but since I’m just trying to force air out at the moment, no free play at all should be more efficient.

It definitely seems like there’s still air trapped in there. Pretty sure I don’t have a leak, since I have seen no brake fluid coming out of anywhere along the lines or on the caliper. And when I ride for a bit, it seems like the engine vibrations knock some air loose, since there’s always a bubble or two when I start bleeding the brakes afterward. Going to keep working and see if I can come up with something.