Disk brake problems

Bonwit

Joined
Jul 20, 2009
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I’ve been having a problem with my front disk brake, which I thought I had solved. Apparently not. The issue is the front brake not releasing. It first happened on the road, and the subsequent overheating ruined the caliper. I found a nice used unit, and installed new stainless pistons, new seals, and of course, new fluid. Everything seemed to be working okay, though honestly, I just rode it a few miles.

Today, I took the bike out after having sat for a few months. Just rolling out of the garage, the brake would not release again. Lots of play in the lever. In order to get it back in the garage, I had to open the bleed screw, which released the brake, though the lever is still sloppy.

I figure I have a master cylinder problem, but having never been inside one, I’m not sure where to start. Back in 2010, I had the master cylinder overhauled and re-sleeved by Fair Spares. Also, the bike has a stainless braided line that is about 35 years old. I’ve heard that on old cars, sometimes a brake hose will collapse internally and create this issue. Any ideas as to where to start would be appreciated.

Thanks!
 
hi. Just a thought…do you have original rubber hose ? I’ve had a hose fail internally trapping pressure on pads…acting as a one way valve. maybe worth a look.
 
My bet is a an issue with the MC vent hole in the cap. If the air space above the rubber diaphragm cannot equalize with atmosphere pressure could build up forcing the fluid down the reservoir ports to caliper.
 
Bonwit said:
snipped Lots of play in the lever. In order to get it back in the garage, I had to open the bleed screw, which released the brake, though the lever is still sloppy.

I figure I have a master cylinder problem, but having never been inside one, I’m not sure where to start. Back in 2010, I had the master cylinder overhauled and re-sleeved by Fair Spares. Also, the bike has a stainless braided line that is about 35 years old. I’ve heard that on old cars, sometimes a brake hose will collapse internally and create this issue. Any ideas as to where to start would be appreciated.

Thanks!
Click to expand...
Odds are your slop in the lever is being caused by the piston in the master cylinder not coming all the way back [retracted]. When was the last time you flushed [not just bled] the brake fluid? This is a normal maintenance item. It should be done every 2 years or less. If you don't, then internal corrosion will start. For $15 you can buy a great brake fluid tester, which can also be used on your cars/trucks. See

Amazon brake fluid tester

Use DOT 4, not DOT 3 fluid, it's better.
 
Its hot down in Texas,
Is your M/C painted black and you left the bike out in the sun to heat it up?
This has caused some M/C to lock up if they were a re-sleeve.
Put a cover over the M/C Like a dew rag or helmet to protect it, while parked from direct sunlight, or take the paint off and leave it in its natural aluminum.
My fix.
Just guessing but maybe???
Tom
 
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Bonwit said:
Also, the bike has a stainless braided line that is about 35 years old. I’ve heard that on old cars, sometimes a brake hose will collapse internally and create this issue.
Click to expand...

Old laminated 'rubber' lines have been known to do so but I've never heard of it happening to a braided line as it's a one-piece PTFE tube.

Bonwit said:
Just rolling out of the garage, the brake would not release again. Lots of play in the lever. I had to open the bleed screw, which released the brake, though the lever is still sloppy.
Click to expand...

"Lots of play in the lever" suggests the master cylinder piston could be stuck in the bore and not fully retracting as chaztuna said.

If the problem was due to the piston not retracting enough to uncover the bleed port to the reservoir then the opposite would be the case that there would be no play at the lever.
 
Two problems have been mentioned here. The brake hose acting as a one way valve. [ in 25 years working as a mechanic, only had to diagnose this problem with one vehicle., It did happen to be a very old EIP Vauxhall ].
Or the master/cylinder is at fault.


The master cylinder can be diagnosed by leaving the cap off and watching the fluid in the reservoir. Squeeze the brake lever and watch the fluid level when you release it. You should be able to tell there is a squirt coming up from the we hole that lets the fluid get into the m/cyl operating side. If you don't see that, chances are the main seal is covering the hole when it returns. It should not do that. That is the only way fluid can get back into the operating side of the the m/cyl allowing for pad wear.
If you see the squirt., then it most probably is the hose, and you have already done what you need to do to diagnose that. [ release the bleed nipple ]

Good luck with your diagnosis.

ps . as an aside, I fondly remember the occasion with the EIP Vauxhall. It was owned by the leader of one of NZ's (small) political parties. It so happened, that another one of my customers came into the shop at the same time, and he was the leader of another (small) Political party.

It was a case of "McGillicudy Serious Party" meets "Private Enterprise Party". Happened about 25 years ago.
 
Tornado said:
My bet is a an issue with the MC vent hole in the cap. If the air space above the rubber diaphragm cannot equalize with atmosphere pressure could build up forcing the fluid down the reservoir ports to caliper.
Click to expand...
That's a good thought, but no joy after removing the cap.
 
CanukNortonNut said:
Its hot down in Texas,
Is your M/C painted black and you left the bike out in the sun to heat it up?
This has caused some M/C to lock up if they were a re-sleeve.
Put a cover over the M/C Like a dew rag or helmet to protect it, while parked from direct sunlight, or take the paint off and leave it in its natural aluminum.
My fix.
Just guessing but maybe???
Tom
Click to expand...
Tom, I've heard about this. It is hot down here for sure, but the bike has been kept in the garage, so I don't think that's it.
 
L.A.B. said:
Old laminated 'rubber' lines have been known to do so but I've never heard of it happening to a braided line as it's a one-piece PTFE tube.



"Lots of play in the lever" suggests the master cylinder piston could be stuck in the bore and not fully retracting as chaztuna said.

If the problem was due to the piston not retracting enough to uncover the bleed port to the reservoir then the opposite would be the case that there would be no play at the lever.
Click to expand...
L.A.B. I think you're right. The problem is the piston getting hung up in the M/C. Now I'm searching for some instruction on how to disassemble it. Perhaps I can smooth the bore with some Scotch Brite. Any thoughts on saving this unit?
 
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kerinorton said:
Two problems have been mentioned here. The brake hose acting as a one way valve. [ in 25 years working as a mechanic, only had to diagnose this problem with one vehicle., It did happen to be a very old EIP Vauxhall ].
Or the master/cylinder is at fault.


The master cylinder can be diagnosed by leaving the cap off and watching the fluid in the reservoir. Squeeze the brake lever and watch the fluid level when you release it. You should be able to tell there is a squirt coming up from the we hole that lets the fluid get into the m/cyl operating side. If you don't see that, chances are the main seal is covering the hole when it returns. It should not do that. That is the only way fluid can get back into the operating side of the the m/cyl allowing for pad wear.
If you see the squirt., then it most probably is the hose, and you have already done what you need to do to diagnose that. [ release the bleed nipple ]

Good luck with your diagnosis.

ps . as an aside, I fondly remember the occasion with the EIP Vauxhall. It was owned by the leader of one of NZ's (small) political parties. It so happened, that another one of my customers came into the shop at the same time, and he was the leader of another (small) Political party.

It was a case of "McGillicudy Serious Party" meets "Private Enterprise Party". Happened about 25 years ago.
Click to expand...
Good tip. I'll give that a try, though I think I already know the result.
 
Richard Tool said:
Return hole in bottom of reservoir blocked - had same issues with mine years ago .
Click to expand...
I have had the same thing happen on a Japanese master cylinder
The screws that retain the cap had corroded
And the corrosion was blocking the return hole
 
Sometimes the pucks which push the pads stick in the bores of the calipers. There is a product called 'rubber grease' which can stop that when applied to the seals on the pucks. When the brake stayed on, you were probably braking for a corner - did you notice the bike becoming much more difficult to steer ?
 
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Disassemble master cylinder and clean it - certain hydraulic fluids get waxy after a time . It was this wax that plugged the return hole .
When it happened to me I had to limp home and crack the bleed screw on the caliper to get the brake to release each time I used the brake .
 
They also corrode from moisture in the system and depends how hard or use your brakes the fluid boils from access heat and moisture in the system, as well old near 50 year brakes time for a complete rebuild or a full upgrade, best thing I ever done was the full Grimica up grade, brakes now work as good as my modern Triumph and you don't want your brakes to fail when you really need them, mine did without any warning, lucky at the time was at slow speed but still got injured and 6 weeks out of action, I rather spend the money as my life is more important than keeping it original looking and that 12" semi floating disc looks so much better on the front wheel.

Ashley
 
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