Master cylinder components

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Master cylinder components


Post by ridermike » Wed Aug 23, 2006 4:36 am

What components are different inside a disk brake master cylinder vs. a juice brake cylinder? I am running the juice brake but after bleeding and bleeding pressure will not stay up. After about ten minutes pedal goes all the way down but after one pump I have plenty of brake. I seem to recall seeing somewhere that the disk MS has a damper or valve in it and I think this might be what I have. I thought I would ask around before taking it down.

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Post by VPH-D » Wed Aug 23, 2006 4:28 pm

The rebuild kits used to include a residual valve. I don't remember for sure, but I think it was for use on the disc brakes. Maybe someone else would chime in...

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Post by RussW » Wed Aug 23, 2006 11:42 pm

Residual pressure valve, which looks like a rubber coated washer, was for the juice brake. If you use it with a disc setup, it will not allow the pistons to retract, and the pads will drag.

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Post by captainharley » Wed Aug 30, 2006 9:14 am

I run stock Harley '74/'75 Superglide disc brakes front/back on my scooter. Rechromed handlebar master cyl on front with DOT-5 fluid.
I have had to only bleed it twice in over 25 years. The "zel" rear master
cyl. has caused a few minor problems over the same time period. I also use DOT-5 in it. It has been so long since I have had to work on the "zel"
that I can not remember if it has the rubber coated washer in it or not. I'm going to say it does not. I will say this that DOT-5 is the only way to go.
AMF :wink: Ride Safe

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Post by rigidpanman » Fri Sep 01, 2006 7:47 pm

the rubber coated washer thing keeps a little bit of pressure in the brake line so the drum brake will actuate like its supposed to,i also agree dot 5 is the way to go.

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My two cents


Post by mogman » Tue Oct 03, 2006 4:07 am

The reason for the valve is used on drum brakes is to keep a little back pressure on the wheel cylinder cups to keep the cups expanded to the cylinder walls to maitain a seal so they will not leak, I do not think however that the lack of this valve would cause the pedal to "go all the way down" after a few minutes. If there is no air AND the brakes are adjusted correctly I can see no mechanism for this action, I could see that if the valve was operating correctly and the brake shoe adjustment was incorrect that the valve may maintain enough pressure for a short period of time to hold the shoes far enough out to work until it bleeds down, I would make sure the shoes were adjusted correctly.
Just my two cents worth and maybe all that it's worth.

BTW it’s absolutely correct that if a drum master cylinder is used on disc brakes it will cause them to drag, heating up the fluid in the caliper making the problem get worse with time, some years ago I purchased a gooseneck trailer with disc brakes, while dragging it home I noticed that the farther I went the more drag I could feel and in a short amount of time the brakes were smoking and one set of duals actually locked up!! It had a vacuum over hydraulic brake system and had been sitting for many months so there was no vacuum in the system to activate the break away feature, I did not have the system hooked up and was very surprised to find pressure on the line when I broke it loose, long story short I got home did some research and found the previous owner installed a master cylinder for a drum system, I removed the valve and have had no more trouble with the brakes for about 17 years now, I also agree to the DOT 5 usage I have some military vehicles that have sat for as long as 7 years at a time and the brakes still work fine, I use DOT 5 in all my brake applications.

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