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maggar

Front Brake Master Cylinder Woes!

10 posts in this topic

Aaaaaghhh!!!!!!

 

Ok, I have completed the restoration of my 241 Progress and I'm over the moon at the results, it's lovely even if I say so myself.

There is however one niggling issue which, try as I might, I simply can't solve.

This bike has the 1986 only weird "semi-hydraulic" set-up on the front brake where a cable pull at the handlebar is transformed into a hydraulic action at a remote master cylinder behind the extended front number board. I'm aware that this is often criticised but I really want the bike to be as original as possible.

 

I am having a nightmare trying to bleed the brake up. It did work if a bit weakly when I picked up the bike from Telford. I have replaced the old pistons and seals in the calliper and given the whole thing a thorough clean (the seals in the calliper ARE the right way up before you ask). 

 

Now initially it appeared as though the seals on the master cylinder piston were past their best so they were replaced. It then appeared that the spring didn't have enough umph to bring the piston back to the "extended" position to allow fluid to drop into the gap in front of it, I tried to space the spring from the end but it didn't work.

I have now located and fitted a completely new piston with new seals and brand new spring and........ it still doesn't work!

 

Brake fluid does pass down the hose okay.

When filled and off the bike squeezing the piston does squirt fluid out from the cylinder (although not much and with little force).

Initially bleeding it seems to work - I have a clear tube connected to the bleed nipple and, to start with, I get a small amount of fluid and air bubbles. I sit and pump the lever for hours and no difference, I seem to have a never ending supply of air and the fluid level in the reservoir hardly drops.

So last night I tried reverse bleeding from the hose bottom up and filled the reservoir that way. Once full I then pulled the lever with the bleed nipple open and the fluid in pipe hardly moves. If I close the bleed nipple I can just about see an almost imperceptible movement of the calliper piston but the pressure doesn't build and the brake doesn't come on AT ALL.

 

Either I'm missing something obvious, or I'm cursed, or fluid is getting past the piston somehow so that it isn't is being forced properly down the hose.

 

Thing is the bike is on Ebay so I need to get it sorted, I placed the ad before "nipping out to the garage to bleed the brake" about 10 days ago.

 

Any thoughts other than can of petrol, Zippo and claim form?

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Keep pumping, don't give up.......

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raise the caliper higher than the master cylinder. then bleed out the trapped air

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I attached a large syringe to the bleed nipple to suck out the air and I keep getting bubbles and no increase in fluid coming out. Is it possible that air is getting into the calliper?

If it is just trapped air then I have a Tardis calliper; I've sucked out about 6 syringes full of air and the calliper is a single pot Grimeca!!!

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If you've got a spare master cylinder try connecting that to the brake line and see if the caliper will bleed properly with a normal master cylinder.

If you can get the caliper to work properly you will only have to reconnect the original master cylinder and reverse bleed that.

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Digger has the right idea. Just like a hydraulic clutch on a car. Remove the whole assembly. Hang the caliper at the top. Forget the hardware, push the piston with a Phillips screwdriver. Best if you had a meter of clear hosed attached to the bleeder held above the caliper to watch the progress. Good luck. Ps. put something between the pads to simulate the disc.

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Thanks, I'll try that tonight.

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Nothing worked!

I decided to make the smaller oil feed hole in the reservoir (through to the cylinder) a bit bigger as it seemed so small compared to the other hole and wasn't allowing any fluid through, so I drilled it out and ...... it worked!

Fed the hose back through the hole in the bottom yoke and bled up the system until I had a solidly braked front wheel - praise the Lord!

So then it was a case of simply attaching the remote master cylinder to its bracket and then to the two holes in the yoke. It was really awkward and I had to slacken off the banjo fitting on the master cylinder to allow the hose to flex and the bracket to line up with the screw holes. I re-tightened the banjo abut it must have let a little air - no problem, I thought just bleed it up again.

Nothing. No air or fluid coming through the clear pipe attached to the bleed nipple again. 

Back to square one.  :wall:

 

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Hmmmnn , The flexable pipe/hose between the two units has deteriorated inside   giving you a check valve/blockage . You changed its position and it worked , returned it to normal and it closed again .  And you can reverse bleed it , but not not from the master out !

 See it more often than you think on car brake hoses ...

Good luck , it is a old hose now ...

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Well after messing about forever with this it works. I altered the depth of the plunger on the actuator arm a few times and bingo. Very strange.

Thanks for all the input everyone.

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