Residual Valve

[datzenmike]

My manuals say to not remove it, and they are kind of delicate. They are a spring loaded valve that keep a few pounds of brake pressure in the lines when the brake peddle is released. This keeps the caliper pads lightly against the rotor to keep them polished clean. The rear brakes are usually drum and a stronger spring is used so it retains more pressure to conteract the return springs on the shoes. This keeps the shoes close to the drum for quick response and much less peddle travel.

 

Anyway I have a spare master and took it apart to look-see.

 

[Z-train]

You tore all the tags off your matresses too-didn't you? :angry:

[mklotz70]

..so I'm guessing the spring is about the only thing different between a disc valve and a drum valve?

[datzenmike]

I would guess. This one is for a drum rear.... I'll pull the front disc one and see if smaller.

[ggzilla]

Disk brake master cylinders don't use a residual valve -- those are for drums only. If you have front disk/rear drum, only the portion of the M/C for the drums has a residual valve. If you convert from all drums to disk/drums you should swap(or at least modify) the M/C.

 

The purpose is as Mike says, the preload the drum brakes with a few psi of pressure. Disk brakes don't want this, but only need a weak pressure which naturally results from the M/C being mounted high on the firewall.

[mklotz70]

Everything I've ever read about residual valves says they're used for cars with the m/c mounted to the frame.....like all the american hotrods. It's to keep the fluid from draining back into the m/c.

[ggzilla]

Mike, you haven't read the Datsun factory service manual?

 

Yes, disk brakes don't normally have a residual valve, but if you mount the M/C below the floor (on the frame for example) then it won't have enough pressure, and you should fit a low-pressure residual valve. Normally the pressure from being high on the firewall is enough to keep the disks floating (very close) next to the rotor.