master brake

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rein
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Postby rein » Sun Mar 18, 2007 4:37 am

Did some miles and for sure the brakes are tip top again with the old 7/8 m/c. Firm padle etc. So it was definitely the valve in the 13/16 m/c.
I will quote from the workshop manual:
The combination inlet and outlet check valve in the head of the cylinder is provided to allow the passage of fluid under pressure from the master piston int the pipe lines, and controls its return into the cylinder, so that a small pressure of approximately 8lb./sq.in. is maintained in the pipelines to ensure that the cups of the wheel cylinders are kept expanded; it also prevents fluid pumped out from the cylinder when 'bleeding' from returning to the cylinder, thus ensuring a fresh charge being deliverd at each stroke of the pedal. End quote.
When I was trying to bleed the system with the faulty valve in the 13/16 m/c I noticed the fluid going back into the system when pedal was returned. There was also no remaining fluid pressure in the system. With the working 7/8 m/c valve the fluid wasn't going back into the system when returning the pedal. Also after applying the brakes with a closed bleeding screw and subsequently opening the bleeding screw, remaining pressure pushed out fluid. (So no air can enter the system via the wheel cylinders.) When this is not happening it can be an indication that the valve in the m/c isn't working and in effect make bleeding the system much more difficult and in my case impossible. A very educational experience!

Cheers,

Rein

alex_holden
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Postby alex_holden » Sun Mar 18, 2007 8:35 am

Interesting, because many people deliberately remove that valve so they can fit front disc brakes.
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If it doesn't work, you're not hitting it with a big enough hammer.

rein
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Postby rein » Sun Mar 18, 2007 9:21 am

I have wondered about that too. I am also not sure why the rear brake cylinders didn't have this problem (allowing air in) to the same extend as the front wheel cylinders did.

Willie
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brakes

Postby Willie » Sun Mar 18, 2007 10:51 am

That is why I will be interested in what thappens when Rein refits the other master cylinder.
Willie
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bmcecosse
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Postby bmcecosse » Sun Mar 18, 2007 11:14 am

So it seems the new cylinder was probably being sold for use with discs - with the valve removed or dis-abled.
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rein
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Postby rein » Sat Mar 31, 2007 10:10 am

For the last time on this subject.
I got the 13/16 M/C back from the brake shop. The pressure retaining valve was working this time. Now bleeding the brakes was straight forward.


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