79 with rear disc. Rebuilt master cylinder and caliper. Bleed it and have good pedel. Ride around and all is good. After I park for a week I have no pedel pressure. I re bled it and all was good. After sitting same thing. No fluid leaking at either end. Where is it getting air from.Same rebuild for front system and it is perfect. Any ideas?
-
Enjoy XS650.com? Consider making a donation to help support the site.
XS650.com receives a small share of sales from some links on this page, but direct donations have a much greater impact on keeping this site going.
jd750ace
Front Toward Enemy
Sounds like your MC seals are not tight enough in the bore and it's bypassing. Will it "pump up" when it's weak?
No it will not pump up unless I rebleed it.
I was thinking maybe the check valve is bypassing. If it has a check valve, i'm not sure
What I dont get is how it could get air in without a fluid leak.
jd750ace
Front Toward Enemy
I don't have the experience with the master to be sure, but is the piston seal on the farthest end of the shaft from actuation a chevron type seal? If so, it can bypass if installed backward, but would still hold up enough to build pedal during bleeding.
jd750ace
Front Toward Enemy
Did you do the cylinder yourself?
Yes I did both at the same time
1974jh5
Curmudgeon.
I've seen that happen with an MG master cylinder. The seal is forced out against the bore by the pressure of the hydraulic fluid. On the return stroke, the seal relaxes. At rest, it's possible for air to seep in around the edges. Is there fluid in the 'bellows' where the pushrod goes in? If so, that's a dead giveaway. I've seen brake master cylinders where the fluid level has dropped inexplicably, when the cylinder is removed from the booster there's fluid in the booster (engine vacuum sucks it out of the rear of the M/C).
Tomterrific
XS650 Junkie
Here is a fast primer on how a MC works. Parts are left out that you should already know. Ignore the caliper as the piston just sitts there imperceptably moving.
There are two holes in the bottom of the reservoir leading to the bore of the MC. The larger hole is feed hole for the back of the piston seal. The extremely tiny hole is in front (closer to the exit) of the piston seal.
As the brake is depressed, the rear, larger hole feeds fluid to the back of the piston. The tiny hole is just in front of the lip of the seal and gets covered as soon as the piston moves.
Releasing the brake has fluid behind the piston seal moving back into the reservoir and feeding around the edges of the seal to make up any volume change from pad wear. The tiny hole gets uncovered when the piston is all the way back. This is extremely important because the heat built up from braking will expand the fluid and the tiny hole is the only way for the pressure to be relieved.
If the holes are blocked the piston cannot move freely. If the tiny hole is blocked, the brake will eventually lock while the bike is moving.
The internal MC spring is what moves the piston and seal back into position.
Air is usually trapped in the system when it is rebuilt and is a bear to remove. I remember my friend, a great amature mechanic, having problems bleeding the air from his Ducati. He mentioned the lever eventually got hard as all the air bled out over time. The trapped air had to come out from the tiny little bleed hole in the MC. After repairing my brakes I need to tap on the system and watch tiny bubbles come out of the little MC hole. Back disk brakes are the worst to bleed.
Check the MC holes to be sure they are clear. If you need to take the MC apart make sure the piston and seals move freely enough for the spring to over come the friction.
Tom
There are two holes in the bottom of the reservoir leading to the bore of the MC. The larger hole is feed hole for the back of the piston seal. The extremely tiny hole is in front (closer to the exit) of the piston seal.
As the brake is depressed, the rear, larger hole feeds fluid to the back of the piston. The tiny hole is just in front of the lip of the seal and gets covered as soon as the piston moves.
Releasing the brake has fluid behind the piston seal moving back into the reservoir and feeding around the edges of the seal to make up any volume change from pad wear. The tiny hole gets uncovered when the piston is all the way back. This is extremely important because the heat built up from braking will expand the fluid and the tiny hole is the only way for the pressure to be relieved.
If the holes are blocked the piston cannot move freely. If the tiny hole is blocked, the brake will eventually lock while the bike is moving.
The internal MC spring is what moves the piston and seal back into position.
Air is usually trapped in the system when it is rebuilt and is a bear to remove. I remember my friend, a great amature mechanic, having problems bleeding the air from his Ducati. He mentioned the lever eventually got hard as all the air bled out over time. The trapped air had to come out from the tiny little bleed hole in the MC. After repairing my brakes I need to tap on the system and watch tiny bubbles come out of the little MC hole. Back disk brakes are the worst to bleed.
Check the MC holes to be sure they are clear. If you need to take the MC apart make sure the piston and seals move freely enough for the spring to over come the friction.
Tom
Thanks for the info Tom I will take it apart tomorrow and check it out. What way does the piston seal face?
Tomterrific
XS650 Junkie
I just happen to have rebuilt an AP Lockheed MC off my Triumph T140 today. The seals face toward the end of the MC where the fluid moves out. If you can think of your hand as a seal with the tips of your spread fingers the sealing lip. The piston would push on the back of your hand/seal, spreading fingers/the lip outward to touch the bore of the MC. The harder the lever is pulled, the more pressure in the system and the harder the lip is forced against the MC bore sealing it. Not my best explanation but perhaps you can figure it out.
Tom
Tom
jd750ace
Front Toward Enemy
If the seal was an umbrella, the bottom of it would face the end of the piston, to capture and "inflate" against the fluid in the line. That expands it and makes it seal.
cooperxs650
XS650 Enthusiast
I had one hell of a time bleeding my rear calliper. It turned out, I was sucking air in the bleeder via the threads. I put some teflon tape on the threads and it bled in no time. Is it possible you're getting air in the system via the threads, and, that's whats making the pedal go soft? It's not a big deal to put some teflon on the threads to seal them and see if that fixes the problem.
