["QUICK-SILVER"]

Originally Posted by Schurkey

They'd be stock on '67--69 (I think) but "custom" on everything else except 'Vettes.

Also original equipment on medium and heavy duty (20~30 series) cheby vans. They had larger wheel cylinders on the rear. Same MC as vettes.

[NEW64OWNER]

Yes, all the Chicom versions of the Corvette master. The ugliest is the version with ports on both sides, although this one is useful on street rods needing a brake light switch.

[NEW64OWNER]

Originally Posted by "QUICK-SILVER"

Also original equipment on medium and heavy duty (20~30 series) cheby vans. They had larger wheel cylinders on the rear. Same MC as vettes.

Can't be the same master as a disc drum van would have a master with an internal residual pressure valve for the rear drums, maybe looks the same though. What year van?

["QUICK-SILVER"]

Originally Posted by NEW64OWNER

What year van?

'77' G20. Carried the same part number as the vettes.

[NEW64OWNER]

Originally Posted by "QUICK-SILVER"

'77' G20. Carried the same part number as the vettes.

looks like the same casting but it's a different master because of the residual pressure valve required for drum brakes, vets have four wheel disc=no rpv

G20 Van part number MC39075
Corvette MC 39052

That's good to know, I prefer the looks of this ,master and didn't know there was one with an RPV for drums

[Tom Vaught]

I read a bit more of this thread and lots of good new info.

So if a "Master Cylinder" is the sum of all of the parts and the "Reservoir Configurations" have to span a lot of applications: Disc/Disc, residual pressure valve required for drum brakes, etc, would you now agree that maybe the "reservoir casting" is a bit more than something to hold Brake Fluid?

Tom V.

[Schurkey]

Far as I know, GM eliminated the residual pressure valve in the master cylinder of drum brakes in '71 or '72. I expect--but cannot confirm--that the aftermarket followed suit shortly thereafter. The RPV was one of "those things" that they discovered wasn't actually needed. GM saved another three cents per vehicle.



Master cylinders come in a bewildering array of part numbers, with a wild combination of features to suit individual applications. Having said that, the reservoir part of the master cylinder--if indeed it is part of the master cylinder, and not remote-mounted; or attached to the master cylinder with nothing more than a couple of O-rings--is something that holds "enough" fluid for the service life of the brake system (barring leaks).

[NEW64OWNER]

GM started using combo valves so an internal RPV wasn't needed at the master

[Firebob]

So without pulling the seat out of the port, how do you determine if the master you have has a RPV installed?

[NEW64OWNER]

You don't pull the seat out on GM masters. You take the piston assy out and the RPV is a thin round metal disc attached to the front of the return spring (very front of assy) The disc is attached by bent over ears and comes off easily by just twisting it slightly. The disc has small holes in a circle. Just remove and don't replace and you no longer have a RPV. This is only for drum/drum masters.
I think your orig post says you have a large and a small reservoir, if so, the large is for disc and will not have an RPV. Connect large reservoir to disc brakes, small to drums.

[NEW64OWNER]

Went back and read your initial post, just plumb large reservoir to your disc and you do not need an additional RPV as the small reservoir will have it internally.

[TedRamAirII]

Originally Posted by Firebob

So without pulling the seat out of the port, how do you determine if the master you have has a RPV installed?

Take a small rod, small enough to go into the seat of the flare. If you feel any resistance, (its rubber with a light spring) the valve is there, if not, its a clear hole.

[NEW64OWNER]

Originally Posted by TedRamAirII

Take a small rod, small enough to go into the seat of the flare. If you feel any resistance, (its rubber with a light spring) the valve is there, if not, its a clear hole.

GM masters don't have that type, Ford uses that style.

[Skip Fix]

Originally Posted by TedRamAirII

Take a small rod, small enough to go into the seat of the flare. If you feel any resistance, (its rubber with a light spring) the valve is there, if not, its a clear hole.

The 67 GTO replacement I have does in front and rear (drum/drum) pulled out easily to do disc/drum

[TedRamAirII]

https://www.google.com/url?sa=i&rct=...68945671649356

[Schurkey]

Originally Posted by TedRamAirII

https://www.google.com/url?sa=i&rct=...68945671649356

I got redirected to:
http://www.camaros.org/brakevalve.shtml

I recognize that we're way off-topic in terms of the first post. However, I'm very unhappy about the way they describe the function of the residual pressure valve. The rest of the article seems accurate.

its function is to maintain 8-16 psi of pressure in the brake system between the master cylinder and drum brake wheel cylinders; this constant low pressure expands the wheel cylinders slightly, pulling the drum brake shoes off their rest stops, and positions the shoes closer to the friction surface of the drum to minimize the pedal travel necessary to engage the brakes. Without these valves, nearly full pedal stroke would be necessary in order to get drum braking action.

While it does retain pressure in the drum-brake hydraulic system, the effect DOES NOT spread the brake shoes from the anchor pin, and it DOES NOT minimize the pedal travel. Brake shoe-to-drum clearance is handled by the brake adjuster mechanism. The residual pressure valve keeps positive fluid pressure on the seals in the wheel cylinder, so that air cannot enter past those seals. The seals are designed to keep fluid IN, not to keep air OUT; they need a little fluid pressure to maintain a gas-tight seal. In normal use, the brake shoes WILL return to the anchor pin when not applied.

[TedRamAirII]

Originally Posted by Schurkey

I got redirected to:
http://www.camaros.org/brakevalve.shtml

I recognize that we're way off-topic in terms of the first post. However, I'm very unhappy about the way they describe the function of the residual pressure valve. The rest of the article seems accurate.


While it does retain pressure in the drum-brake hydraulic system, the effect DOES NOT spread the brake shoes from the anchor pin, and it DOES NOT minimize the pedal travel. Brake shoe-to-drum clearance is handled by the brake adjuster mechanism. The residual pressure valve keeps positive fluid pressure on the seals in the wheel cylinder, so that air cannot enter past those seals. The seals are designed to keep fluid IN, not to keep air OUT; they need a little fluid pressure to maintain a gas-tight seal. In normal use, the brake shoes WILL return to the anchor pin when not applied.

I agree, that statement didn't make sense to me, you are dead on. I did like the picture of the RPV that another member stated was a FORD design, wtf? I have pulled them out myself, on my Firebird.

[Firebob]

Just to keep air from pulling past the seals was the reason I was given for the use of the RPV as well. The thing I don't understand is, if I never had one before, why do I all of a sudden need one now? I'm sure the original master didn't have one.

[Skip Fix]

Originally Posted by Schurkey

I got redirected to:
http://www.camaros.org/brakevalve.shtml

I recognize that we're way off-topic in terms of the first post. However, I'm very unhappy about the way they describe the function of the residual pressure valve. The rest of the article seems accurate.


While it does retain pressure in the drum-brake hydraulic system, the effect DOES NOT spread the brake shoes from the anchor pin, and it DOES NOT minimize the pedal travel. Brake shoe-to-drum clearance is handled by the brake adjuster mechanism. The residual pressure valve keeps positive fluid pressure on the seals in the wheel cylinder, so that air cannot enter past those seals. The seals are designed to keep fluid IN, not to keep air OUT; they need a little fluid pressure to maintain a gas-tight seal. In normal use, the brake shoes WILL return to the anchor pin when not applied.

I always read it was to reduce pedal travel taking up the slack in the wheel cylinders since the shoe's springs push them back and disc calipers piston./pad has less travel to engage friction material. Interesting.

[TedRamAirII]

Originally Posted by Skip Fix

I always read it was to reduce pedal travel taking up the slack in the wheel cylinders since the shoe's springs push them back and disc calipers piston./pad has less travel to engage friction material. Interesting.

The springs on the shoes, pull the shoes back until they contact the top anchor pin. The little pressure the RPV's hold isnt enough to keep the shoes off the pin. Then the pistons cant go "in" any further, the RPV keeps a little pressure on the cylinder cups to keep them from pulling in air. They only seal one way. Some cylinders have metal cones on the springs that help keep them sealed to the cylinders. That's why you keep your brakes adjusted (with automatic adjusters) so there is very little pedal travel. Whenever you have a lot of pedal travel, the brake adjustment is the first thing to verify.