Master cylinder advice

[Iron Andy]

Hi,

you may have read that my front brake seized on and through me off my bike.

I have completely dismantled the front brake and believe the problem is in the master cylinder. There are two holes at the bottom of the reservoir. The smaller one of the two, when the components are installed is cover by the rubber cup that fits on the spring, and uncovered when the lever is pulled in.

Is this correct, how does the brake fluid return after the brake has been released, through the smaller or larger hole?

When I bled the brake it was very hard to get the fluid to pass to the caliper. I had to use pneumatic pressure to bleed the brake, via one of those easy bleeds that attaches the valve on the inner tube.

Thanks in advance,

Andy

[grbrown]

Andy,

It is a long time since I owned my shovel, but I did cut my teeth on bleeding brakes with the blooming thing! I had cowhorn bars on mine, so the master cylinder pointed upwards and the brake pipe was a metal pipe which created a high point in the system, just forward of the cylinder. So bleeding involved moving fluid uphill from the master cylinder. If your bike has this setup then pressure bleeding was a wise move! I made a flat plate with a nipple in it to fit the master cylinder, so I could use a pressure bleeder.

If the internal parts of the master cylinder are all clean and in good condition and the brake functions OK now, it seems you have it cracked.

[Limey_Dave]

The front hole (nearest the brake line) should be clear with the brake off.
When you pull the lever the cup seal moves forward past the hole and spreads to hold the pressure.The hole may then look to be covered by the back of the cup seal depending how far it has moved.
When the lever is released the cup will move back behind the front hole and fluid can return to the resevoir and pressure removed.
If you had problems moving fluid when bleeding and brake is now holding pressure it points to the spring not pushing the cup far enough back.
A rebuild kit will include a new spring aswell as seals and piston and should sort the problem.
Pressure bleeding can overcome and mask this fault by blowing fluid down the rear hole and past the cup from behind.

[spinlondon]

The fluid enters the system and returns through the hole nearest the pipe. The other hole allows fluid in to fill the space left by the piston.
When the lever is released, the piston pushes the fluid back up in to the resevoir. Sometimes when the fluid is low or you release the lever too quickly, you will see the fluid fountain up in the resevoir.

[Iron Andy]

Hi all,

I think I've found the problem with the MC. The metal plunger that goes from the lever into the ali. plunger inside the bore of the MC, had a small spring on it, therefore the internal components where sat approx. 8mm from where they should have been.

I am still concerned about the feel of the brake. I used the syringe method, bleeding from the caliper upto the MC. What a waste of time. For me I could not get the syringe or the piece of plastic pipe free of air bubbles, and when I took the pipe out of the bottle of fluid, the fluid just ran out the bottom of the pipe, giving me even more air bubbles. What am I doing wrong?

I reverted back to to the pneumatic method, much better results but the lever still feels very spongy. I am going to buy some more DOT 5 and have another go. This DOT 5 seems to be very sensitive, does it take long for it to settle down in the brake system after the brakes have been bled?

Will be pulling the clutch and gear box apart in the next couple of days to check what has happend after my little tumble, after the front brake locked on

Thanks everyone,

Andy.

[grbrown]

Andy,

I haven't found DOT5 to be any more difficult to use. If you have air in the system, it will naturally rise to the highest spot in the system. So having done your best, consider ways you can persuade the silly bubbles to find their way into the reservoir, hence out of the system.

I assume you have a single disc, in which case this shouldn't be too difficult, as you should also have a single run of pipe between the reservoir and calliper. I have faffed around enough in the past on my FX, so can sympathise!

Start at the calliper. Make sure the pipe fitting is at the top of the calliper. If need be, undo the fixing bolts to achieve this and tap the calliper and pipe with a screwdriver handle or something, the purpose being to dislodge any bubbles inside. Tap the pipe along its length, when the majority of any trapped air should be at the top of the pipe. Any interim pipe fixings are candidates for air bubbles.

Turn the handlebars and/or reservoir, to ensure the reservoir is the highest point. Gently and repeatedly apply the brake lever, which should admit air from the pipe into the reservoir. If there is as much as you indicate, this will be noticeable. You may need to top up the fluid level.

Restore everything you've disturbed and hopefully the brake should be improved. If it is safe to ride, do so along as bumpy a road as you can, when any other trapped air will be dislodged upwards and repeat as before, to get it out of the system.

Hope that helps. You've brought back distant and not entirely pleasant memories! Let me know when you are going to tackle the back brake and I will leave the country....