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Tool Talk Discussion Forum

Question about master cylinders

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ldj

07-15-2006 18:29:32




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I am restoring a 1957 Isetta and the parts are either nonexistent or very expensive. For that reason you either make it or find a part from another car that will work. On the master cylinder piston, the end that goes in first there are 6 small holes around the edge. I found a mastercylinder from a motorcycle that is almost exactly the same and it will work. The motorcycle master cylinder has a little 6 pointed piece of metal afixed to the end and each of the six points cover the 6 holes. Looks like they are using the metal for a check valve or maybe you could call it a reed valve. Now a rubber cup goes in first right against the check valve, or in case of the original piston, the holes. My thinking is the rubber cup would do the same thing as the metal, keep fluid from passing through the holes one way but let it go the other. Anyone have any idea of the difference. Should I leave it as is or should I remove the piece of metal?
L.D.

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jmixigo

07-17-2006 07:15:58




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 Re: Question about master cylinders in reply to ldj, 07-15-2006 18:29:32  
What you describe is the "residual check valve" incorporated into master cylenders designed for disc brake systems. Disc brakes don"t really retract they simply release with an absence of pressure. If the Isetta has drum brakes (most likely) then what will happen when you get a good pedal is dragging or locked brakes-unless you take out the "little 6 point piece"-those holes let the springs retract the shoes from the drums.

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Not a Brake Specialist

07-17-2006 18:02:20




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 Re: Question about master cylinders in reply to jmixigo, 07-17-2006 07:15:58  

I respectfully disagree with some of your statements.

Most DRUM brake systems DO require residual pressure valves, while most DISC brake circuits do NOT require rpv's.

The residual pressure valve in a DRUM brake system prevents air ingestion into the wheel cylinders when the brake pedal is released.



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Roy Suomi

07-16-2006 07:21:53




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 Re: Question about master cylinders in reply to ldj, 07-15-2006 18:29:32  
If you want to keep the original look , I've used a company called Apple Hydraulics to rebuild a master cylinder for a Chev 1-1/2 ton truck. They bore and reline with bress sleeve to original size. Worked good.I found them in Hemmings Magazine..



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Not a Brake Specialist

07-16-2006 05:40:26




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 Re: Question about master cylinders in reply to ldj, 07-15-2006 18:29:32  
I believe your are thinking correctly.

IMHO, I would use the bike piston as is. ....If there is a problem, then you could remove the metal "reed valve" later, but I do not think that will be necessary.

I'm guessing that the bike m/c is a newer-than-1957 design, and there was a good reason for its existence.



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