Brake Master Cylinder

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cjr198car
Posts: 122
Joined: 25 Mar 2009 10:11
Location: Minneapolis Minnesota

Re: Brake Master Cylinder

Postby cjr198car » 26 Jul 2011 22:15

I meant to post this here and not on the other archive post with regards to the brake assembly switch...

More info...

I was told that Im releasing pressure from the front brakes (bleeder on MC closest to the firewall), but this is releasing pressure from my rear brakes...

?

mstarr68
Posts: 3
Joined: 20 Aug 2011 22:38

Re: Brake Master Cylinder

Postby mstarr68 » 21 Aug 2011 16:17

In regards to the pressure build up in the rear brake system in the previous post.

Most people don't realize, that even though you bought a master cylinder for a 280zx, which will be used on a 4 wheel disc brake system, may not be the right master cylinder for your setup. You need to check out the check valves in the bottom of the body. A master cylinder for a rear drum setup as a different style check valve than one for a rear disc setup. I have seen this on several different master cylinders.

If you pull off the "big nuts" at the bottom of the master cylinder, underneath there will be a round black plastic/rubber check valve. They should both have a "hole" in the middle of it to let pressure back into the master cylinder (which pulls the disc brake caliper back away from the rotor. In a drum setup, one of the check valves will NOT have any holes in the middle. This keeps the fluid back in the lines for a drum setup, but interferes with proper operation of a disc setup.

You may want to check this out, and make sure you don't have a master cylinder meant for a rear drum brake setup. If so, then get a rebuild kit for a 4 wheel disc setup, and replace this check valve with the one with a hole in the middle.

This exact thing happened to me when I upgraded my 510 to 4 wheel disc setup. I had several extra master cylinders laying around that I was able to compare, and found this out. I replaced the one check valve and everything has worked since.

Hope this helps a person or two.

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defdes
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Joined: 12 May 2005 14:45
Location: Vermont

Re: Brake Master Cylinder

Postby defdes » 21 Aug 2011 16:27

^^^That's a very interesting factoid that I have never heard about before. Thanks for the tip.

datzenmike
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Location: Van. Isle.

Re: Brake Master Cylinder

Postby datzenmike » 21 Aug 2011 19:57

mstarr68 wrote:In regards to the pressure build up in the rear brake system in the previous post.

Most people don't realize, that even though you bought a master cylinder for a 280zx, which will be used on a 4 wheel disc brake system, may not be the right master cylinder for your setup. You need to check out the check valves in the bottom of the body. A master cylinder for a rear drum setup as a different style check valve than one for a rear disc setup. I have seen this on several different master cylinders.

If you pull off the "big nuts" at the bottom of the master cylinder, underneath there will be a round black plastic/rubber check valve. They should both have a "hole" in the middle of it to let pressure back into the master cylinder (which pulls the disc brake caliper back away from the rotor. In a drum setup, one of the check valves will NOT have any holes in the middle. This keeps the fluid back in the lines for a drum setup, but interferes with proper operation of a disc setup.

You may want to check this out, and make sure you don't have a master cylinder meant for a rear drum brake setup. If so, then get a rebuild kit for a 4 wheel disc setup, and replace this check valve with the one with a hole in the middle.

This exact thing happened to me when I upgraded my 510 to 4 wheel disc setup. I had several extra master cylinders laying around that I was able to compare, and found this out. I replaced the one check valve and everything has worked since.

Hope this helps a person or two.



This isn't quite as I understand it but close. Both front and rear systems have a residual pressure valve that is spring loaded to keep several pounds of pressure in the lines at all times. Generally disc brakes have several pounds to gently keep the pads against the rotors to keep them clean. A master with a drum rear brake has a residual valve with a stronger spring to retain slightly higher pressure in the rear lines. This keeps the rear shoes from being fully retracted by the return springs and reduces excess peddle travel to move them up against the drums. I replaced my 710 (rear drum) master with a zx (rear disc) master and they both had the rubber valves and springs. I couldn't tell if the zx springs were 'stiffer' on the back or not the difference is so slight.

In the picture below, the brake line fits into the top of the fitting on the left and seats against the middle cone shaped thing (valve seat) with the hole in it. The rubber valve sitting on top of the valve spring sits below the cone with the hole. The spring is such that any pressure above several pounds will push down on the spring and open the rubber valve and fluid flows back into the master reservoir. Depending on the spring rate several pound of pressure will be 'trapped' in the lines.

Image

Image

If you run a rear disc master with rear drums the residual pressure will possibly be too low and the peddle may be mushy or the rear brakes may not be working as hard as they could because they have too far to travel.
If you were to run a rear drum master with a rear disc the residual pressure may be too high causing the rear disc pads to rub harder than necessary and drag, wearing out sooner or overheating possibly.
"Nissan 'shit the bed' when they made these, plain and simple." McShagger510 on flattop SUs


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